Method and apparatus for diagnosis and treatment of arrhythmias

ABSTRACT

An implantable anti-arrhythmia device such as a defibrillator or anti-tachycardia pacemaker with an associated patient activator. The arrhythmia detection function is divided between the patient and the implanted device, rather than merely providing patient activation or a patient override. The implantable device is provided with a tachyarrhythmia recognition mechanism, which operates essentially continuously, and which, upon meeting a first set of criteria, will trigger delivery of an anti-arrhythmia therapy such as pacing, cardioversion or defibrillation. The patient is provided with an activator which informs the implanted device that the patient believes that anti-arrhythmia therapy is necessary. In response to receipt of the activation signal, the implanted device defines a time interval thereafter during which a second, less stringent set of arrhythmia detection criteria must be met, in response to which the device will deliver a cardioversion or defibrillation pulse.

BACKGROUND OF THE INVENTION

This invention relates to devices which detect and/or treattachyarrhythmias (rapid heart rhythms), and more specifically, tomechanisms to distinguish among various tachyarrhythmias and to provideappropriate therapies to treat the identified tachyarrhythmias.

In conjunction with implantable cardioverters and defibrillators,numerous mechanisms for communication between the patient and theimplanted device have been proposed. Particularly in conjunction withimplantable defibrillators or cardioverters, the provision of a patientwarning signal indicating that cardioversion or defibrillation is aboutto commence, is disclosed in U.S. Pat. No. 4,210,149 issued to Heilmanet al, U.S. Pat. No. 5,332,400 issued to Alferness and U.S. Pat. No.5,190,034 issued to Sholder. The warning signals provided to the patienttypically take the form of audio signals or electrical stimulation usingelectrodes associated with the implanted device.

In addition to communication from the implanted device to the patient,communication and control signals from the patient to the device arealso known to the art. For example, U.S. Pat. No. 5,443,486 issued toHrdlicka et al and U.S. Pat. No. 4,365,633 issued to Loughman et al,both disclose devices allowing the patient to program or control theoperation of the implanted device. Particularly in conjunction withimplanted defibrillators, the warning and control functions have beenintegrated, so that the patient is provided with a warning signal priorto defibrillation or cardioversion, and with a mechanism for overridingthe implanted device, to prevent delivery of an undesired or unneededcardioversion or defibrillation pulse. Such systems are disclosed inU.S. Pat. No. 5,190,034 issued to Sholder and U.S. Pat. No. 4,210,149issued to Heilman et al, cited above. In such systems, the implanteddevice monitors the patient's heart rhythm to determine the presence ofan arrhythmia requiring cardioversion or defibrillation. Having detectedsuch a rhythm, the device provides the warning signal indicating theimminent delivery of a cardioversion or defibrillation shock, which thepatient may then override. Allowing the patient to initiate delivery ofa cardioversion or defibrillation shock is also known to the art. Forexample, U.S. Pat. No. 3,952,750 issued to Mirowski et al disclosesatrial implantable cardioverter in which the patient initiates deliveryof an atrial cardioversion pulse, and U.S. Pat. No. 5,498,062 issued toAdams et al discloses an implantable defibrillator in which the patientinitiates operation of the arrhythmia detection mechanism, to determinewhether delivery of a cardioversion or defibrillation pulse isnecessary.

Presently available implantable anti-arrhythmia devices employsophisticated arrhythmia detection and classification methods toaccurately determine whether delivery of therapy is appropriate.Particularly in the context of devices such as cardioverters anddefibrillators which have the potential to induce arrhythmias if notappropriately synchronized to the patient's heart rhythm, thesedetection methods tend to be conservative, in order to avoid delivery ofunnecessary therapy. In such cases, it may sometimes take the implanteddevice longer than the patient to determine that delivery of a therapyis needed. Patient activators as discussed above which trigger therapyon request address this problem, but do not provide for the possibilityof patient error. The device described in the Adams patent cited abovedeals with the possibility of patient error by determining whethertherapy is appropriate after a request by the patient, but employs thesame set of criteria for patient requested therapy as for deviceinitiated therapy, and thus may not provide for therapies as quickly oras often as may be desirable in response to patient's requests. Thepresent invention is believed to offer the patient the ability toquickly and safely receive therapy in response to a request, whenappropriate.

SUMMARY OF THE INVENTION

The present invention is directed toward an implantable anti-arrhythmiadevice such as a defibrillator or anti-tachycardia pacemaker with anassociated patient activator. The arrhythmia detection function isdivided between the patient and the implanted device, rather than merelyproviding patient activation or a patient override. In the presentinvention, the implantable device is provided with a tachyarrhythmiarecognition mechanism, which operates essentially continuously, andwhich, upon meeting a first set of criteria, will trigger delivery of ananti-arrhythmia therapy such as pacing, cardioversion or defibrillation.In addition, the patient is provided with an activator which informs theimplanted device that the patient believes that anti-arrhythmia therapyis necessary. In response to receipt of the activation signal, theimplanted device defines a time interval thereafter during which asecond, less stringent set of arrhythmia detection criteria must be met,in response to which the device will deliver a cardioversion ordefibrillation pulse. By this mechanism, more stringent criteria aredefined for self-activation of the device than for patient activation,which is believed to enhance the accuracy and flexibility of the systemin dealing with tachyarrhythmias as compared to the systems describedabove.

In a preferred embodiment of the invention, the particular therapy to beinitiated is programmed into the implanted device, and takes priorityover automatic, device-initiated therapies. In the preferred embodimentdisclosed herein, the patient-activated therapy is directed towardstermination of atrial tachyarrhythmias. However, the invention generallyis believed applicable in the context of detection and termination ofventricular arrhythmias as well. In the particular embodiment disclosedherein, an atrial anti-arrhythmia therapy is delivered in response tothe patient's request for therapy only if the heart's rhythm meetscriteria consistent with atrial fibrillation or atrial tachycardia,within a predetermined time interval, e.g., one minute, after thepatient-activated therapy was requested. In addition, thepatient-activated therapy is disabled on occurrence of a variety ofevents, including detection of termination of the atrial fibrillation oratrial tachycardia episode or a determination that sustained atrialfibrillation or tachycardia has persisted for more than a pre-determinedperiod. In response to the patient activation signal, the implanteddevice notifies the activator whether an atrial rhythm appropriate fortreatment is present and whether a therapy is available for delivery inresponse to the patient's request.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment of an implantablepacemaker/cardioverter/defibrillator of a type appropriate for use inpracticing the present invention, in conjunction with a human heart.

FIG. 2 illustrates a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator in which the invention may bepracticed.

FIG. 3 illustrates an exemplary embodiment of a patient activator whichmay be used in practicing the present invention.

FIG. 4 is a functional block diagram of the patient activator of FIG. 3.

FIG. 5 illustrates the basic timing intervals employed by a preferredembodiment of the present invention to classify sequences of heartevents.

FIG. 6 illustrates the classification system employed by a preferredembodiment of the present invention to classify sequences of heartevents.

FIG. 7 is a table illustrating the operation of a continuous recognitionmachine employed by a preferred embodiment of the present invention toaccomplish classification of heart event sequences according to thesystem illustrated in FIG. 4.

FIG. 8 is a table illustrating the operation of a continuous recognitionmachine employed by a preferred embodiment of the present invention toidentify the probable occurrence of normal sinus rhythm or sinustachycardia based upon series of heart event sequences as classifiedusing the continuous recognition machine illustrated in FIG. 5.

FIG. 9 is a table illustrating the operation of a continuous recognitionmachine employed by a preferred embodiment of the present invention toidentify the probable occurrence of normal sinus rhythm or sinustachycardia in the presence of far field R-wave sensing in the atrium,based upon series of heart event sequences as classified using thecontinuous recognition machine illustrated in FIG. 5.

FIG. 10 is a table illustrating the operation of a second continuousrecognition machine employed by a preferred embodiment of the presentinvention to identify the probable occurrence of atrial fibrillation orflutter based upon series of heart event sequences as classified usingthe continuous recognition machine illustrated in FIG. 5.

FIG. 11 is a table illustrating the operation of a continuousrecognition machine employed by a preferred embodiment of the presentinvention to identify the probable occurrence of AV nodal tachycardiabased upon series of heart event sequences as classified using thecontinuous recognition machine illustrated in FIG. 5.

FIG. 12 is a functional flowchart illustrating the operation of theheart rhythm classification methodology employed by the presentinvention.

FIG. 13 is a functional flowchart illustrating the interaction of thevarious rules for initiation and prevention of anti-arrhythmiatherapies.

FIG. 14 is a diagram illustrating the operation of the atrialfibrillation/atrial tachycardia evidence counter.

FIG. 15 is a functional flowchart illustrating the operation of thepatient activator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a defibrillator and lead set according to the presentinvention. The ventricular lead includes an elongated insulative leadbody 16, carrying three concentric coiled conductors, separated from oneanother by tubular insulative sheaths. Located adjacent the distal endof the lead are a ring electrode 24, an extendable helix electrode 26,mounted retractably within an insulative electrode head 28, and anelongated coil electrode 20. Each of the electrodes is coupled to one ofthe coiled conductors within the lead body 16. Electrodes 24 and 26 areemployed for cardiac pacing and for sensing ventricular depolarizations.At the proximal end of the lead is a bifurcated connector 14 whichcarries three electrical connectors, each coupled to one of the coiledconductors. The defibrillation electrode 20 may be fabricated fromplatinum, platinum alloy or other materials known to be usable inimplantable defibrillation electrodes and may be about 5 cm in length.

The atrial/SVC lead includes an elongated insulative lead body 15,carrying three concentric coiled conductors, separated from one anotherby tubular insulative sheaths, corresponding to the structure of theventricular lead. Located adjacent the J-shaped distal end of the leadare a ring electrode 21 and an extendable helix electrode 17, mountedretractably within an insulative electrode head 19. Each of theelectrodes is coupled to one of the coiled conductors within the leadbody 15. Electrodes 17 and 21 are employed for atrial pacing and forsensing atrial depolarizations. An elongated coil electrode 23 isprovided, proximal to electrode 21 and coupled to the third conductorwithin the lead body 15. Electrode 23 preferably is 10 cm in length orgreater and is configured to extend from the SVC toward the tricuspidvalve. In one preferred embodiment tested by the inventors,approximately 5 cm of the right atrium/SVC electrode was located in theright atrium, with the remaining 5 cm located in the SVC. At theproximal end of the lead is a bifurcated connector 13 which carriesthree electrical connectors, each coupled to one of the coiledconductors.

The coronary sinus lead includes an elongated insulative lead body 6,carrying one coiled conductor, coupled to an elongated coileddefibrillation electrode 8. Electrode 8, illustrated in broken outline,is located within the coronary sinus and great vein of the heart. At theproximal end of the lead is a connector plug 4 which carries anelectrical connector, coupled to the coiled conductor. The coronarysinus/great vein electrode 8 may be about 5 cm in length.

An implantable pacemaker/cardioverter/defibrillator 10 is shown incombination with the leads, with the lead connector assemblies 4, 13 and14 inserted into the connector block 12. Optionally, insulation of theoutward facing portion of the housing 11 of thepacemaker/cardioverter/defibrillator 10 may be provided using a plasticcoating, for example parylene or silicone rubber, as is currentlyemployed in some unipolar cardiac pacemakers. However, the outwardfacing portion may instead be left uninsulated, or some other divisionbetween insulated and uninsulated portions may be employed. Theuninsulated portion of the housing 11 optionally serves as asubcutaneous defibrillation electrode, used to defibrillate either theatria or ventricles. Other lead configurations and electrode locationsmay of course be substituted for the lead set illustrated. For example,atrial defibrillation and sensing electrodes might be added to eitherthe coronary sinus lead or the right ventricular lead instead of beinglocated on a separate atrial lead, allowing for a two-lead system.

FIG. 2 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator in which the present invention mayusefully be practiced. This diagram should be taken as exemplary of thetype of device in which the invention may be embodied, and not aslimiting, as it is believed that the invention may usefully be practicedin a wide variety of device implementations, including devices providingtherapies for treating atrial arrhythmias instead of or in addition toventricular arrhythmias, cardioverters and defibrillators which do notprovide anti-tachycardia pacing therapies, anti-tachycardia pacers whichdo not provide cardioversion or defibrillation, and devices whichdeliver different forms of anti-arrhythmia therapies such nervestimulation or drug administration.

The device is provided with a lead system including electrodes, whichmay be as illustrated in FIG. 1. Alternate lead systems may of course besubstituted. If the electrode configuration of FIG. 1 is employed, thecorrespondence to the illustrated electrodes is as follows. Electrode311 corresponds to electrode 11, and is the uninsulated portion of thehousing of the implantable pacemaker/cardioverter/defibrillator.Electrode 320 corresponds to electrode 20 and is a defibrillationelectrode located in the right ventricle. Electrode 310 corresponds toelectrode 8 and is a defibrillation electrode located in the coronarysinus. Electrode 318 corresponds to electrode 28 and is a defibrillationelectrode located in the superior vena cava. Electrodes 324 and 326correspond to electrodes 24 and 26, and are used for sensing and pacingin the ventricle. Electrodes 317 and 321 correspond to electrodes 19 and21 and are used for pacing and sensing in the atrium.

Electrodes 310, 311, 318 and 320 are coupled to high voltage outputcircuit 234. Electrodes 324 and 326 are coupled to the R-wave amplifier200, which preferably takes the form of an automatic gain controlledamplifier providing an adjustable sensing threshold as a function of themeasured R-wave amplitude. A signal is generated on R-out line 202whenever the signal sensed between electrodes 324 and 326 exceeds thepresent sensing threshold.

Electrodes 317 and 321 are coupled to the P-wave amplifier 204. whichpreferably also takes the form of an automatic gain controlled amplifierproviding an adjustable sensing threshold as a function of the measuredR-wave amplitude. A signal is generated on P-out line 206 whenever thesignal sensed between electrodes 317 and 321 exceeds the present sensingthreshold. The general operation of the R-wave and P-wave amplifiers 200and 204 may correspond to that disclosed in U.S. Pat. No. 5,117,824, byKeimel, et al., issued Jun. 2, 1992, for an Apparatus for MonitoringElectrical Physiologic Signals, incorporated herein by reference in itsentirety.

Switch matrix 208 is used to select which of the available electrodesare coupled to wide band (0.5-200 Hz) amplifier 210 for use in digitalsignal analysis. Selection of electrodes is controlled by themicroprocessor 224 via data/address bus 218, which selections may bevaried as desired. Signals from the electrodes selected for coupling tobandpass amplifier 210 are provided to multiplexer 220, and thereafterconverted to multi-bit digital signals by A/D converter 222, for storagein random access memory 226 under control of direct memory accesscircuit 228. Microprocessor 224 may employ digital signal analysistechniques to characterize the digitized signals stored in random accessmemory 226 to recognize and classify the patient's heart rhythmemploying any of the numerous signal processing methodologies known tothe art.

Telemetry circuit 330 receives downlink telemetry from and sends uplinktelemetry to the patient activator by means of antenna 332. Data to beuplinked to the activator and control signals for the telemetry circuitare provided by microprocessor 224 via address/data bus 218. Receivedtelemetry is provided to microprocessor 224 via multiplexer 220. Theparticular telemetry system employed is not critical to practicing theinvention, and any of the numerous types of telemetry systems known foruse in implantable devices may be used. In particular, the telemetrysystems as disclosed in U.S. Pat. No. 5,292,343 issued to Blanchette etal., U.S. Pat. No. 5,314,450, issued to Thompson, U.S. Pat. No.5,354,319, issued to Wyborny et al. U.S. Pat. No. 5,383,909, issued toKeimel, U.S. Pat. No. 5,168,871, issued to Grevious, U.S. Pat. No.5,107,833 issued to Barsness or U.S. Pat. No. 5,324,315, issued toGrevious, all incorporated herein by reference in their entireties, aresuitable for use in conjunction with the present invention. However, thetelemetry systems disclosed in the various other patents cited hereinwhich are directed to programmable implanted devices, or similar systemsmay also be substituted. The telemetry circuit 330 is of course alsoemployed for communication to and from an external programmer, as isconventional in implantable anti-arrhythmia devices.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies, and, for purposes ofthe present invention may correspond to circuitry known in the priorart. An exemplary apparatus is disclosed for accomplishing pacing,cardioversion and defibrillation functions as follows. The pacertiming/control circuitry 212 includes programmable digital counterswhich control the basic time intervals associated with DDD, VVI, DVI,VDD, AAI, DDI and other modes of single and dual chamber pacing wellknown to the art. Circuitry 212 also controls escape intervalsassociated with anti-tachyarrhythmia pacing in both the atrium and theventricle, employing, any anti-tachyarrhythmia pacing therapies known tothe art.

Intervals defined by pacing circuitry 212 include atrial and ventricularpacing escape intervals, the refractory periods during which sensedP-waves and R-waves are ineffective to restart timing of the escapeintervals and the pulse widths of the pacing pulses. The durations ofthese intervals are determined by microprocessor 224, in response tostored data in memory 226 and are communicated to the pacing circuitry212 via address/data bus 218. Pacer circuitry 212 also determines theamplitude of the cardiac pacing pulses under control of microprocessor224.

During pacing, the escape interval counters within pacer timing/controlcircuitry 212 are reset upon sensing of R-waves and P-waves as indicatedby signals on lines 202 and 206, and in accordance with the selectedmode of pacing on time-out trigger generation of pacing pulses by paceroutput circuits 214 and 216, which are coupled to electrodes 317, 321,324 and 326. The escape interval counters are also reset on generationof pacing pulses, and thereby control the basic timing of cardiac pacingfunctions, including anti-tachyarrhythmia pacing.

The durations of the intervals defined by the escape interval timers aredetermined by microprocessor 224, via data/address bus 218. The value ofthe count present in the escape interval counters when reset by sensedR-waves and P-waves may be used to measure the durations of R--Rintervals, P--P intervals, PR intervals and R-P intervals, whichmeasurements are stored in memory 226 and used in conjunction with thepresent invention to diagnose the occurrence of a variety oftachyarrhythmias, as discussed in more detail below.

Microprocessor 224 operates as an interrupt driven device, and isresponsive to interrupts from pacer timing/control circuitry 212corresponding to the occurrences of sensed P-waves and R-waves andcorresponding to the generation of cardiac pacing pulses. Theseinterrupts are provided via data/address bus 218. Any necessarymathematical calculations to be per-formed by microprocessor 224 and anyupdating of the values or intervals controlled by pacer timing/ controlcircuitry 212 take place following such interrupts. Microprocessor 224includes associated ROM in which the stored program controlling itsoperation as described below resides. A portion of the memory 226 (FIG.2) may be configured as a plurality of recirculating buffers, capable ofholding series of measured intervals, which may be analyzed in responseto the occurrence of a pace or sense interrupt to determine whether thepatient's heart is presently exhibiting atrial or ventriculartachyarrhythmia.

The arrhythmia detection method of the present invention may includeprior art tachyarrhythmia detection algorithms. As described below, theentire ventricular arrhythmia detection methodology of presentlyavailable Medtronic pacemaker/cardioverter/defibrillators is employed aspart of the arrhythmia detection and classification method according tothe disclosed preferred embodiment of the invention. However, any of thevarious arrhythmia detection methodologies known to the art, asdiscussed in the Background of the Invention section above might alsousefully be employed in alternative embodiments of the invention.

In the event that an atrial or ventricular tachyarrhythmia is detected,and an anti-tachyarrhythmia pacing regimen is desired, appropriatetiming intervals for controlling generation of anti-tachyarrhythmiapacing therapies are loaded from microprocessor 224 into the pacertiming and control circuitry 212, to control the operation of the escapeinterval counters therein and to define refractory periods during whichdetection of R-waves and P-waves is ineffective to restart the escapeinterval counters. Alternatively, circuitry for controlling the timingand generation of anti-tachycardia pacing pulses as described in U.S.Pat. No. 4,577,633, issued to Berkovits et al on Mar. 25, 1986, U.S.Pat. No. 4,880,005, issued to Pless et al on Nov. 14, 1989, U.S. Pat.No. 7,726,380, issued to Vollmann et al on Feb. 23, 1988 and U.S. Pat.No. 4,587,970, issued to Holley et al on May 13, 1986, all of which areincorporated herein by reference in their entireties may also be used.

In the event that generation of a cardioversion or defibrillation pulseis required, microprocessor 224 employs the escape interval counter tocontrol timing of such cardioversion and defibrillation pulses, as wellas associated refractory periods. In response to the detection of atrialor ventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, microprocessor 224 activates cardioversion/defibrillation controlcircuitry 230, which initiates charging of the high voltage capacitors246, 248 via charging circuit 236, under control of high voltagecharging control line 240. The voltage on the high voltage capacitors ismonitored via VCAP line 244, which is passed through multiplexer 220 andin response to reaching a predetermined value set by microprocessor 224,results in generation of a logic signal on Cap Full (CF) line 254,terminating charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse is controlled by pacertiming/control circuitry 212. Following delivery of the fibrillation ortachycardia therapy the microprocessor then returns the device tocardiac pacing and awaits the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization.

One embodiment of an appropriate system for delivery and synchronizationof ventricular cardioversion and defibrillation pulses and forcontrolling the timing functions related to them is disclosed in moredetail in commonly assigned U.S. Pat. No. 5,188,105 by Keimel, issuedFeb. 23, 1993, and incorporated herein by reference in its entirety. Ifatrial defibrillation capabilities are included in the device,appropriate systems for delivery and synchronization of atrialcardioversion and defibrillation pulses and for controlling the timingfunctions related to them may be found in PCT Patent Application No.W092/18198 by Adams et al., published Oct. 29, 1992, and in U.S. Pat.No. 4,316,472 by Mirowski et al., issued Feb. 23, 1982, bothincorporated herein by reference in their entireties. In addition, highfrequency pulse bursts may be delivered to electrodes 317 and 321 toterminate atrial tachyarrhythmias, as described in PCT PatentPublication No. W095/28987, filed by Duffin et al and PCT PatentPublication No. W095/28988, filed by Mehra et al, both incorporatedherein by reference in their entireties.

However, any known cardioversion or defibrillation pulse controlcircuitry is believed usable in conjunction with the present invention.For example, circuitry controlling the timing and generation ofcardioversion and defibrillation pulses as disclosed in U.S. Pat. No.4,384,585, issued to Zipes on May 24, 1983, in U.S. Pat. No. 4,949,719issued to Pless et al, cited above, and in U.S. Pat. No. 4,375,817,issued to Engle et al, all incorporated herein by reference in theirentireties may also be employed.

In the illustrated device, delivery of the cardioversion ordefibrillation pulses is accomplished by output circuit 234, undercontrol of control circuitry 230 via control bus 238. Output circuit 234determines whether a monophasic or biphasic pulse is delivered, whetherthe housing 311 serves as cathode or anode and which electrodes areinvolved in delivery of the pulse. An example of output circuitry fordelivery of biphasic pulse regimens may be found in the above citedpatent issued to Mehra and in U.S. Pat. No. 4,727,877, incorporated byreference in its entirety.

An example of circuitry which may be used to control delivery ofmonophasic pulses is set forth in commonly assigned U.S. Pat. No.5,163,427, by Keimel, issued Nov. 17, 1992, also incorporated herein byreference in its entirety. However, output control circuitry asdisclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al on Sep. 4,1990 or U.S. Pat. No. 4,800,883, issued to Winstrom on Jan. 31, 1989both incorporated herein by reference in their entireties, may also beused in conjunction with a device embodying the present invention fordelivery of biphasic pulses.

In modern implantable cardioverter/defibrillators, the particulartherapies are programmed into the device ahead of time by the physician,and a menu of therapies is typically provided. For example, on initialdetection of an atrial or ventricular tachycardia, an anti-tachycardiapacing therapy may be selected and delivered to the chamber in which thetachycardia is diagnosed or to both chambers. On redetection oftachycardia, a more aggressive anti-tachycardia pacing therapy may bescheduled. If repeated attempts at anti-tachycardia pacing therapiesfail, a higher level cardioversion pulse may be selected thereafter.Therapies for tachycardia termination may also vary with the rate of thedetected tachycardia, with the therapies increasing in aggressiveness asthe rate of the detected tachycardia increases. For example, fewerattempts at anti-tachycardia pacing may be undertaken prior to deliveryof cardioversion pulses if the rate of the detected tachycardia is abovea preset threshold. The references cited above in conjunction withdescriptions of prior art tachycardia detection and treatment therapiesare applicable here as well.

In the event that fibrillation is identified, high frequency burststimulation as discussed above may be employed as the initial attemptedtherapy. Subsequent therapies may be delivery of high amplitudedefibrillation pulses, typically in excess of 5 joules. Lower energylevels may be employed for cardioversion. As in the case of currentlyavailable implantable pacemakers/cardioverter/defibrillators, and asdiscussed in the above-cited references, it is envisioned that theamplitude of the defibrillation pulse may be incremented in response tofailure of an initial pulse or pulses to terminate fibrillation. Priorart patents illustrating such pre-set therapy menus ofanti-tachyarrhythmia therapies include the above-cited U.S. Pat. No.4,830,006, issued to Haluska, et al., U.S. Pat. No. 4,727,380, issued toVollmann et al. and U.S. Pat. No. 4,587,970, issued to Holley et al.

FIG. 3 illustrates the general physical configuration of a patientactivator of the type which may be employed with the present invention.The activator 100 generally takes the form of a plastic enclosureprovided with a push button 102 by which the patient may requestdelivery of the predefined patient-initiated therapy. The device isbattery powered, employing batteries accessible by means of the batterycover 104. On the reverse side of the device, not visible, are twoindicator lights, one green, one amber, which are used to provideinformation to the patient with regard to the status and functioning ofthe patient-initiated therapy.

FIG. 4 is a block functional diagram of a patient activator of the typeappropriate for use in conjunction with the present invention. Thisdevice corresponds generally to patient activators presently availablecommercially for use in conjunction with implanted Medtronic pacemakers,and in particular, corresponds generally to the Medtronic Model-9462patient activator presently in commercial distribution for use inconjunction with implanted bradycardia pacers. Control functions areprovided by microprocessor 108, based upon programming stored in itsassociated read-only memory located therein. Microprocessor 108 providesoutput signals for producing audible patient alert signals by means ofdriver 110 and speaker 112. Microprocessor 108 also provides controlsignals to LED driver 114 to power the associated amber and greencolored LEDs 116, referred to above. The device is powered by a battery118 which is coupled to the microprocessor 108 by means ofpower/switching/battery monitor circuitry 120, which also provides themicroprocessor with an indication that push button 122 has been pressed.

Communication with microprocessor 108 is accomplished by means of theantenna driver/switching circuit 124, the receiver demodulator 126 andRF antenna 128. Transmissions from the implanted device are received byantenna 128, and are demodulated by receiver demodulator 126 to beprovided to the microprocessor. In response to received transmissionsfrom the implanted device, the microprocessor controls operation of theaudio and light drivers 110 and 114 to indicate the nature of thecommunication received. Transmissions to the implanted device, forexample, in response to activation of the push button 102 are providedby microprocessor 108 to the antenna drive/switching circuit, which thencommunicates with the implanted device by means of antenna 128.

1. Arrhythmia detection for device initiated therapy

The implanted device employs a sophisticated arrhythmia detection andclassification methodology to initiate delivery of anti-arrhythmiatherapy. However, in some circumstances the patient may be aware of thepresence of tachyarrhythmia before the implanted device has confirmedits presence, and may employ the activator to request delivery of ananti-arrhythmia therapy. In such case, the implanted device employs aless stringent set of criteria to determine whether therapy isappropriate, and communicates its status to the patient by means of thelights and tones provided by the activator.

The arrhythmia detection and classification system normally employed bythe implanted device according to the present invention to triggerdelivery of anti-arrhythmia therapy may be any of the numerousarrhythmia detection methodologies employed to trigger delivery ofanti-arrhythmia therapy in the patents cited herein. In the preferredembodiment of the present invention disclosed herein, the implanteddevice employs a prioritized set of inter-related rules for arrhythmiadetection. Each rule contains a set of one or more "clauses" which mustbe satisfied (criteria which must be met). While all clauses of a ruleare satisfied, the rule is indicated to be met. In the context of thepresent application this is referred to as the rule "firing". It ispossible for multiple rules to be "firing" at the same time, with thehighest priority rule taking precedence. Some rules trigger, delivery oftherapy when firing. Other rules inhibit delivery of therapy whenfiring. The highest priority rule firing at any specific time controlsthe behavior of the device. For example, the firing of a rule whichtriggers therapy is superseded by the firing of higher priority rulespreventing delivery of therapy. Rules cease firing when their clausescease to be satisfied, whether or not a therapy is triggered by therule.

Each rule includes a set of clauses or criteria which, when satisfied,indicate the likely occurrence of a specified type of heart rhythm,including various tachyarrhythmias, sinus tachycardia and normal sinusrhythm. A specific rhythm or tachyarrhythmia may have more than oneassociated rule. The rules are interrelated, such that progress towardmeeting the requirements of a clause of one rule may also be the subjectmatter of a clause of a different rule.

The specific criteria set forth by the clauses of the various rules asdisclosed include a number of known criteria for evaluating heartrhythm, including the entire arrhythmia detection and classificationsystem employed in the presently available Medtronic 7219 pacemakercardioverter defibrillators, as well as criteria disclosed in U.S. Pat.No. 5,330,508, issued to Gunderson, as will be discussed below. Inaddition, a number of new evaluation criteria are included within theclauses of various rules. One such new detection methodology is basedupon the classification of the events occurring associated with thesequence of two ventricular depolarizations into a limited number ofevent patterns, based upon the number and times of occurrences of atrialevents, preceding the two most recent ventricular events. An eventpattern is developed for each individual ventricular event, so thatsuccessive event patterns overlap one another. The inventors havedetermined that certain sequences of event patterns are stronglyindicative of specific types of heart rhythms. For heart rhythms ofwhich this is true, a defined set of indicative event pattern sequencesor a "grammar" is defined. Adherence of the heart rhythm to the grammarsassociated with various heart rhythms is determined by simultaneouslyoperating continuous recognition machines, the outputs of which form thesubject matter of one or more clauses, within the hierarchy of rules.

In a preferred embodiment of the invention, the device is provided withrules which when satisfied indicate the presence of sustained atrialfibrillation and sustained atrial flutter and in response to detectionthereof delivers anti-atrial fibrillation or anti-atrial tachycardiatherapies. These rules include a set of various new classificationcriteria, including an atrial fibrillation/atrial tachycardia evidencecounter which is incremented and decremented on a beat by beat basis andcompared with a defined threshold count or counts taken as indicative ofatrial fibrillation or atrial tachycardia. The atrial rate andregularity is also monitored and atrial fibrillation or atrialtachycardia is preliminarily detected when the evidence counter is at orabove such a threshold and the atrial rhythm meets defined rate zonecriteria associated with atrial fibrillation or atrial tachycardia. Whenboth the evidence count and the rate zone criteria are met, thearrhythmia underway is preliminarily determined to be atrialfibrillation or atrial tachycardia, depending on which rate zonecriteria are met. A sustained atrial fibrillation/atrial tachycardiaduration timer is then initiated and continues to time until terminationof atrial tachyarrhythmia is detected. The time duration since thepreliminary detection of an atrial tachyarrhythmia is continuallycompared to one or more minimum duration values associated with theatrial tachyarrhythmia determined to presently be underway and/or thenext scheduled therapy for such arrhythmia. If the time duration sincepreliminary detection of atrial arrhythmia meets or exceeds theapplicable minimum duration value, and other associated criteria arealso met, the next scheduled anti-atrial arrhythmia therapy isdelivered.

Additional associated criteria which must be met as a prerequisite todelivery of atrial anti-tachyarrhythmia therapies may include expirationof a minimum interval from the most recently delivered therapy notfollowed by a detected termination of atrial tachyarrhythmia,confirmation that the most recent heart cycles do not indicate a returnto sinus rhythm, time duration since preliminary detection of atrialtachyarrhythmia being less than a maximum duration value, time of daycorresponding to a predefined time range and/or less than a presetnumber of atrial anti-arrhythmia therapies having been delivered in apreceding time period.

With each ventricular event, the timing of atrial and ventricular eventsoccurring during the preceding two R--R intervals is analyzed to developa "pattern code". FIG. 5 illustrates the various defined time intervals,employed to develop the pattern codes. Each of the two R--R intervals isdivided into four zones, in which zone 1 encompasses the first 50milliseconds following the ventricular event initiating the R--Rinterval, zone 2 extends from the end of zone 1 until halfway throughthe R--R interval. Zone 3 extends from halfway through the R--R intervalto 80 milliseconds prior to the ventricular event ending the R--Rinterval and zone 4 includes the last 80 milliseconds of the R--Rinterval.

In order to determine the pattern codes, each individual R--R intervalis assigned a "beat code", based on the number of occurrence of atrialevents during the R--R interval, and their location with regard to thefour defined zones. Three criteria are evaluated in order to assign eachR--R interval with a beat code, including the number of atrial eventsoccurring during the R--R interval, referred to as the "P count", theduration of the R-P interval associated with the R--R interval, and theduration of the P-R interval associated with the R--R interval. The R-Pinterval is the time in milliseconds from the beginning ventricularevent in the RR interval to the first atrial event occurring within theinterval, if any. The P-R interval is the time in milliseconds from thelast atrial event in the R--R interval, if any, to the concludingventricular event in the R--R interval. It should be noted that ifmultiple atrial events occur during the R--R interval, the sum of theR-P and P-R intervals will not equal the R--R interval. Based on the Pcount and the times of occurrence of the atrial depolarizations, a beatcount of zero to nine is generated. The algorithm for generating thebeat code is as follows.

If P count equals 1 and an atrial event occurs in zone 3, the beat codeis zero. If P count equals 1 and the atrial event occurs in zone 1, thebeat code is 1. If P count equals 1 and the atrial event occurs in zone4, the beat code is 2. If P count equals 1 and the atrial event occursin zone 2, the beat code is 3.

If P count equals 2, and an atrial event occurs in zone 3 but not zone1, the beat code is 9. If P count equals 2 and an atrial event occurs inzone 3 and in zone 1, the beat code is 4. If P count equals 2 and atrialevents occur in zones 1 and 4, the beat code is 5. All other R--Rintervals containing two atrial events result in a beat code of 6.

If P count is greater than or equal to 3, the beat code is 8. If P countis equal to 0, the beat code is 7.

Given 10 beat codes, it would be expected that 100 corresponding patterncodes for two R--R interval sequences would be generated. However, theinventors have determined that the library of event patterns mayusefully be reduced substantially, and have derived a set of 18 patterncodes as illustrated in FIG. 6. In the illustrations, two successiveR--R intervals are illustrated, with downward extending lines indicativeof ventricular events and upward extending lines indicative of atrialevents. Zone 1 is illustrated as a short horizontal bar extending fromthe first ventricular event in each R--R interval. Zone 4 is illustratedas a short horizontal bar extending back from the last ventricular eventin each R--R interval. A vertically extending dotted line is indicativeof the dividing line between zone 2 and zone 3, halfway through the R--Rinterval, upwardly extending lines, coupled to the horizontal base lineare indicative of atrial events occurring in the specific zoneillustrated. Upwardly extending lines which float above the base lineare indicative of atrial events that may occur in either of the twozones to which they are adjacent.

Pattern code A, corresponding to a beat code pair (0,0) is a patterncode sinus tachycardia.

Pattern code B, corresponding to beat code (0,7) arises, among othertimes, when a premature ventricular contraction occurs and is detectedprior to the next atrial depolarization.

Pattern code C corresponds to beat code pairs (7,4) or (7,9), andarises, among other times, in the aftermath of isolated PVC'S.

Pattern code D, corresponding to beat code pairs (0,4) or (0,9) arises,among other times, when an isolated premature atrial contraction occurs,with no corresponding ventricular event.

Pattern code E, corresponding to beat code pairs (4,0) or (9,0) arises,among other times, in the aftermath of an isolated PAC, with resumptionof normal sinus rhythm.

Pattern code F, corresponding to beat code pair (1,1) arises, amongother times, during a junctional rhythm, with the atrial depolarizationsbeing detected closely following depolarizations in the ventricles. Italso arises in disassociated rhythms in which the atria and ventriclesbeat independently, but slightly out of phase.

Pattern code G, corresponding to beat code pair (2,2) arises, amongother times, when a rhythm has a junctional origin, with ventriculardepolarizations detected just slightly after atrial depolarizations. Italso arises in disassociated rhythms in which atria and ventricle beatindependently at close to the same rate, but slightly out of phase.

Pattern code H, corresponding to beat code pair (5,7) arises, amongother times, in junctional rhythms in which atrial and ventriculardepolarizations are sensed closely spaced to one another, but in noconsistent time order.

Pattern code 1, corresponding to beat code pair (7,5) and pattern codeJ, corresponding to beat code pair (7,1) are both employed forrecognition of AV nodal reentrant tachycardia.

Pattern code K, corresponding to beat code pair (2,7) arises, amongother times during nodal rhythms, as well as ventricular tachycardia,ventricular fibrillation and ventricular flutter, but rarely, if at all,occurs in cases of atrial fibrillation.

Pattern code L, corresponding to beat code (0,2) occasionally arises incases of dual tachycardia, in which the atria and ventricles are beatingindependently, but out of phase.

Pattern code M, beat code pair (2,0) also arises in these situations.

Pattern code N, corresponding to beat code pair (3,3) arises in cases ofventricular tachycardia with one to one retrograde conduction.

Pattern code 0 is a default pattern code, based on the failure of thepattern code to correspond to any of codes A-N, above, with theadditional requirement that the P count for the first R--R interval is 1and the P count for the second R--R interval is 2. This (pattern codearises frequently in atrial fibrillation, among other rapid atrialrhythms. Pattern code P is also a default pattern code, designated ifthe beat code pair does not correspond to any of the beat code pairsdesignated in conjunction with pattern codes A-N, above, with a P countfor the first R--R interval of 2 and a P count for the second R--Rinterval of 1.

Pattern code Q is a default pattern code assigned in response to beatcode pairs which do not correspond to any of pattern codes A-N above, inwhich both P counts are 2. Like pattern codes O and P, this pattern codeis indicative of atrial fibrillation, and/or rapid atrial rhythms.

Pattern Code Y is a default pattern code assigned to all beat code pairsnot falling into any of previously defined pattern codes A-Q, in whichthere is at least one atrial event in each R--R interval, and the sum ofthe two P counts exceeds 3. Pattern code Z is a default pattern codeassigned to all beat code pairs not corresponding to any of patterncodes A-Y above.

While the above rules appear to be complex, they may be veryconveniently implemented by means of a look up table, as set forth inFIG. 7, which assigns each of the 100 possible beat code pairs to one ofthe designated pattern codes. By use of the look up table stored inmemory, the microprocessor within the device can readily and rapidlydetermine the appropriate pattern code associated with each successiveventricular event. These pattern codes can be stored as numbers, asindicated in parentheses in FIG. 6, and their order analyzed by means ofa software implemented continuous recognition machine to determinewhether the sequences of pattern codes correspond to defined grammarscorresponding to specific arrhythmias or groups of arrhythmias. Theoperation of the continuous recognition machines in order to accomplishthis result is discussed in more detail, below. However, for purposes ofunderstanding the general operation of the device, in conjunction withthe functional flowcharts of FIG. 13, it need only be understood thatthe continuous recognition machines output a count indicative of thedegree of correspondence of the sensed rhythm to the defined grammarsfor each arrhythmia, and that the rules for identifying the variousarrhythmias include clauses setting forth criteria against which theoutput counts of the continuous recognition machines are compared.

Several of the rules employ continuous recognition machines implementedby the microprocessor, which applies sequences of pattern codes or beatcodes, as they are generated with each ventricular event, to anassociated look-up table. Each look up table defines a set of sequentialstates, indicated by bracketed numbers, beginning with the reset state0!, and a set of other defined states, arranged horizontally across thetable. Possible pattern codes or beat codes are listed vertically. Inoperation, with each ventricular event, the processor determines itspresent state and the most recent pattern or beat code. Based on thetable, the processor transitions to the next state, and awaits the nextpattern or beat code. As long as the pattern or beat codes adhere to thedefined grammar for the rhythm in question, the reset state is avoided.Adherence to the defined grammar over an extended sequence of beats isdetermined by means of a corresponding count, which may be incrementedwith each pattern or beat code adhering to the grammar, and may be resetto zero or decremented in response to pattern or beat codes which do notadhere to the grammar as indicated by a return to the reset state 0!.The current count for each continuous recognition machine is comparedagainst a defined threshold value in one or more clauses, in one or morerules.

The continuous recognition machine for recognition of sinus tachycardiaand normal sinus rhythm employs the look-up table of FIG. 8, using botha strict adherence to grammar (basic behavior) and less a less strictadherence to the grammar (exponential decay), with transitions betweenthe two types of counter behavior defined according to the rules setforth below. The continuous recognition machine for sinus tachycardiaand normal sinus rhythm employs a count, "CRMedST" which is incremented,up to a maximum count, e.g. 13, in response to each transition to anon-reset state (or in response to the first R--R interval after apower-on reset or other device reset, where the pattern code isunknown). On each ventricular event, all CRM counts are updated by theprocessor and compared against applicable recognition threshold values.The value of CRMedST is compared to its corresponding CRM thresholdvalue, e.g. 6, in a clause of the rule for recognizing sinustachycardia.

If the pattern code associated with the present beat resets thecontinuous recognition machine of FIG. 8, and the counter behavior ispresently set to "basic behavior", CRMedST is reset to 0. If the patterncode associated with the present beat resets the continuous recognitionmachine of FIG. 6, and the counter behavior is presently set to"exponential decay", CRMedST is decremented by the CRMedST decrementamount. If after decrementing, CRMedST is then less than 0, the counterbehavior is set to "basic behavior" and CRMedST is set to 0. If afterdecrementing, CRMedST is greater than 0, then the CRMedST decrementamount is set to either twice the present decrement amount or to thedecremented value of CRMedST, whichever is less. By this mechanism, theamount of the decrement increases a factor of two with each successivefailure to meet the pattern grammar, hence an exponential decay of thevalue of CRMedST with successive failures to meet pattern grammar.

If the pattern code associated with the present beat does not reset thecontinuous recognition machine of FIG. 8 or is unknown, the value ofCRMedST is incremented by 1, up to the maximum of 13. If the CRMedSTcounter behavior is set to "basic behavior", and the incremented valueof CRMedST is greater than or equal to the associated CRM thresholdvalue, e.g. 6, then CRMedST counter behavior is set to "exponentialdecay" and the CRMedST decrement amount is set to 2. If the CRMedSTcounter behavior is set to "exponential decay", and the incrementedvalue of CRMedST equals the maximum count the CRMedST decrement amountis set to 2.

FIG. 9 illustrates the look-up table employed in conjunction with thecontinuous recognition machine for recognizing beat code sequencescorresponding to normal sinus rhythm or to sinus tachycardia in thepresence of far field R-wave sensing in the atrium. The rules forincrementing and decrementing the associated count CRMedSTFR correspondto those for incrementing and decrementing the value of CRMedST, asdiscussed above.

If the beat code associated with the present beat resets the continuousrecognition machine of FIG. 9, and the counter behavior is presently setto "basic behavior", CRMedSTFR is reset to 0. If the beat codeassociated with the present beat resets the continuous recognitionmachine of FIG. 9, and the counter behavior is presently set to"exponential decay", CRMedSTFR is decremented by the CRMedSTFR decrementamount. If after decrementing, CRMedSTFR is then less than 0, thecounter behavior is set to "basic behavior" and CRMedSTFR is set to 0.If after decrementing CRMedSTFR is greater than 0, then the CRMedSTFRdecrement amount is set to either twice the present decrement amount orto the decremented amount of CRMedSTFR, whichever is less.

If the beat code associated with the present beat does not reset thecontinuous recognition machine of FIG. 9 or is unknown, the value ofCRMedSTFR is incremented by 1, up to the maximum count, e.g. 13. If theCRMedSTFR counter behavior is set to "basic behavior", and theincremented value of CRMedSTFR is greater than or equal to theassociated CRM threshold value, e.g. 6, then CRMedSTFR counter behavioris set to "exponential decay" and the CRMedST decrement amount is set to2. If the CRMedSTFR counter behavior is set to "exponential decay", andthe incremented value of CRMedSTFR equals the maximum count the CRMedSTdecrement amount is set to 2.

FIG. 10 is a look-up table employed by the CRM used to detect the likelyoccurrence of atrial fibrillation or flutter. The Count associated withthe CRM is designated "CRMAL". The value of CRMAL is employed in aclause of a rule for recognizing atrial fibrillation or flutter. Thiscontinuous recognition machine requires strict adherence to the patterngrammar. The value of CRMAL is incremented by one up to the maximumcount, e.g. 13, in response to any pattern code that does not reset thecontinuous recognition machine, and is reset to 0 whenever thecontinuous recognition machine is reset.

FIG. 11 is a look-up table employed by the CRM used to detect the likelyoccurrence of atrial-ventricular nodal tachycardia. The Count associatedwith the CRM is designated "CRMAVNRT". The value of CRMAVNRT is employedin a clause of a rule for recognizing AV nodal reentrant tachycardia.The value of CRMAVNRT is incremented by one up to the maximum count,e.g. 13, in response to any pattern code that does not reset thecontinuous recognition machine, and is reset to 0 whenever thecontinuous recognition machine is reset.

In addition to adherence to the defined grammars as set forth above, therules of the present invention also employ rate and interval basedrecognition criteria presently employed by the Medtronic Model 7219implantable pacemaker/cardioverter/defibrillator. These criteria arediscussed in detail in U.S. Pat. No. 5,342,402, issued to Olson,incorporated herein by reference in its entirety. These criteria arealso discussed below.

Presently available pacemaker-cardioverter-defibrillator devices, suchas the Model 7219 PCD devices available from Medtronic, Inc., employprogrammable fibrillation interval ranges and tachycardia detectioninterval ranges. In these devices, the interval range designated asindicative of fibrillation consists of intervals less than aprogrammable interval (VFDI) and the interval range designated asindicative of ventricular tachycardia consists of intervals less than aprogrammable interval (VTDI) and greater than or equal to VFDI. R--Rintervals falling within these ranges are measured and counted toprovide a count (VTEC) of R--R intervals falling within the ventriculartachycardia interval range and a count (VFEC) of the number intervals,out of a preceding series of a predetermined number (FEB) of intervals,which fall within the ventricular fibrillation interval range. VTEC isincremented in response to R--R intervals that are greater than or equalto VFDI but shorter than VTDI, is reset to zero in response to intervalsgreater than or equal to VTDI and is insensitive to intervals less thanVFDI. VTEC is compared to a programmed value (VTNID) and VFEC iscompared to a corresponding programmable value (VFNID). When one of thecounts equals its corresponding programmable value, the device diagnosesthe presence of the corresponding arrhythmia, i.e. tachycardia orfibrillation and delivers an appropriate therapy, e.g. anti-tachycardiapacing, a cardioversion pulse or a defibrillation pulse. In addition,the physician may optionally require that the measured R--R intervalsmeet a rapid onset criterion before VTEC can be incremented and can alsooptionally require that should a rate stability criterion fail to bemet, VTEC will be reset to zero. If the device is further programmed toidentify the occurrence of a fast ventricular tachycardia, detection ofventricular fibrillation or tachycardia according to the above methodserves as a provisional detection, which may be modified, as discussedbelow. An exemplary set of parameters might be VFDI=320 ms, VFNID=18/24preceding intervals, VTDI=400 ms, VTNID=16 intervals.

In addition to the tachycardia and fibrillation detection criteria(VTEC>=VTNID, VFEC>=VFNID) discussed above, detection of tachycardia orfibrillation detection may also be optionally accomplished using acombined count of all intervals indicative of tachycardia orfibrillation. This combined count (VFEC+VTEC) is compared to a combinedcount threshold (CNID). If VTEC+VFEC is equal or -greater than CNID, thedevice checks to see whether VFEC is at least a predetermined number(e.g. 6). If so, the device checks to determine how many of a number(e.g. 8) of the immediately preceding intervals are greater or equal toVFDI. If a predetermined number (e.g. 8) are greater than or equal toVFDI, tachycardia is detected, otherwise ventricular fibrillation isdetected. If the device is further programmed to identify the occurrenceof a fast ventricular tachycardia, detection of ventricular fibrillationor tachycardia according to the above method serves as a provisionaldetection, which may be modified, as discussed below.

In addition, the model 7219 PCD is provided with a method ofdistinguishing a fast ventricular tachycardia from either ventricularfibrillation or slow ventricular tachycardia. In conjunction with fastventricular tachycardia detection, the physician determines whetherdetection of a fast ventricular tachycardia is to be accomplishedfollowing a provisional diagnosis of ventricular tachycardia, followinga provisional diagnosis of ventricular fibrillation, or followingeither. If detection of fast ventricular tachycardia is enabled, thenfollowing provisional detection of ventricular tachycardia orfibrillation, as discussed above, the immediately preceding measuredintervals are examined to determine whether the provisional detection offibrillation or tachycardia should be confirmed or amended to indicatedetection of fast ventricular tachycardia.

If fast ventricular tachycardia detection following a provisionaldetection of ventricular tachycardia is enabled, a value VFTDImax isdefined, which is greater than or equal to VFDI. If fast ventriculartachycardia detection following a provisional detection of ventricularfibrillation is enabled, a value VFTDImin,, is defined, which is lessthan or equal to VFDI. If ventricular tachycardia is provisionallydetected, intervals less than VFTDImax are taken as indicative of fastventricular tachycardia. If ventricular fibrillation is provisionallydetected, intervals greater than or equal to VFTDImin. are taken asindicative of fast ventricular tachycardia.

If fibrillation was provisionally detected, the device may require thatat least 7 or all 8 of the preceding 8 intervals fall within the fastventricular tachycardia interval range (greater than or equal toVFTDImin) to detect fast ventricular tachycardia. Otherwise, theprovisional detection of ventricular fibrillation is confirmed. Ifventricular tachycardia is provisionally detected, the device may onlyrequire that at least 1 or 2 of the preceding 8 intervals fall withinthe fast ventricular tachycardia interval range (less than VFTDImax inorder to detect fast ventricular tachycardia. Otherwise, the provisionaldetection of (slow) ventricular tachycardia is confirmed.

The entire arrhythmia detection methodology of the Model 7219 PCD is notretained in the disclosed embodiment of the present invention, in thatthe above described criteria for detecting fast ventricular tachycardiaare not employed, with the criteria for detecting ventriculartachycardia and ventricular fibrillation employed as the two lowestpriority rules for triggering delivery of ventricularanti-tachyarrhythmia therapies. However, the fast tachycardiarecognition criteria described above could readily be added if desired,in which case, the criteria for detection of ventricular fibrillation,fast ventricular tachycardia and ventricular tachycardia according tothis methodology would comprise the three lowest priority rules employedfor detection of ventricular tachyarrhythmia.

The arrhythmia detection and classification scheme of the presentinvention also employs a measurement of R--R interval variability, asdisclosed in U.S. Pat. No. 5,330,508 issued to Gunderson andincorporated herein by reference in its entirety. R--R intervalvariability is measured by the processor sorting the 12-18 previousmeasured R--R intervals into bins in RAM, each bin being 10 ms in width,spanning the range of 240 ms through 2019 ms. The sum (RR Modesum) ofthe numbers of intervals in the two bins individually having the highestnumbers of intervals is calculated and compared against preset thresholdvalues. The higher the value of RR Modesum, the lower the variability ofRR intervals, and the more likely the rhythm is a monomorphicventricular tachycardia. The RR Modesum is compared against variousthreshold values in clauses of rules for detecting ventriculartachycardia, ventricular tachycardia in the presence of supraventriculartachycardia, atrial fibrillation or flutter, and AV nodal reentranttachycardia. A buffer of 18 measured intervals is also provided in RAM.Intervals less than 240 ms do not appear in the bins, but are loaded inthe buffer. Following detection initialization or power on reset, thebuffer is cleared, and thereafter intervals are entered in the buffer.If fewer than 12 intervals are in the buffer, the value of RR Modesum isdefined as "unknown". If 12 or more intervals are in the buffer, RRModesum is equal to the fraction defined by the number of intervalsstored in the buffer residing in the two bins having the highest numbersof intervals divided by the number of intervals in the buffer. Forexample, if the RR Modesum threshold is set at 0.75, then RR Modesums of9/12, 12/16, 14/18, etc. would meet the threshold.

In conjunction with the operation of rules intended to identify thelikely occurrence of ventricular and supraventricular tachycardia, themicroprocessor also keeps track of the number of R--R intervals whichlikely contain sensed atrial events caused by far field R-waves, out ofa preceding series of R--R intervals. If an R--R interval is determinedlikely to contain a far field R-wave, the Far Field R-wave Criterion ismet for that R--R interval. The microprocessor determines that an eventsensed in the atrium is likely a far field R-wave, according to thefollowing methodology.

The microprocessor maintains a Far RP buffer in RAM containing the eightmost recent R-P intervals less than 160 ms and a Far PR buffercontaining the eight most recent P-R intervals less than 60 ms. Inresponse to the occurrence of R--R interval having a P count equal to 2,the R-P and P-R intervals for the R--R interval are compared to fixedthresholds. For example, the processor may check to determine whetherthe P-R interval is less than or equal to 60 milliseconds or whether theR-P interval is less than or equal to 160 milliseconds. It should bekept in mind that in conjunction with an R--R interval having a P countof 2, the R-P interval is measured between the ventricular eventinitiating the R--R interval and the first occurring atrial event andthe P-R interval is measured between the second to occur atrial eventand the ventricular event ending the R--R interval.

If the P-R interval is less than or equal to 60 milliseconds, theprocessor subtracts the shortest P-R interval (PRmin) in the Far PRbuffer from the longest (PRmax). If the value of the difference is lessthan or equal to 30 milliseconds, the processor compares the P--Pinterval between the two atrial events during the R--R interval underconsideration with the P--P interval separating the first atrial eventin the R--R interval in consideration from the last atrial event in theproceeding R--R interval. If the difference between these two values isgreater than or equal to 30 milliseconds, the processor subtracts thecurrent P-R interval from the average (PRave) of the P-R intervals inthe buffer. If the absolute value of the difference is less than adefined Far R Stability value, e.g. 20 ms, the R--R interval underconsideration likely includes a far field R-wave and the Far FieldR-Wave Criterion is met.

Similarly, if the measured R-P interval in the R--R interval underquestion is less than or equal to 160 milliseconds, the processorsubtracts the, shortest (RPmin) of the eight R-P intervals in the Far RPbuffer from the longest (RPmax) R-P interval in the buffer if thedifference is less than or equal to 50 ms, the processor compares theP--P interval in the R--R interval under question with the P--P intervalseparating the final atrial event of the preceding R--R interval to thefirst atrial event of the R--R interval under question. If, as discussedabove, the difference between the two PP intervals is greater than orequal to 30 milliseconds, the processor subtracts the current R-Pinterval from the average (RPave) of the R-P intervals in the buffer. Ifthe absolute value of the difference is less than the Far R Stabilityvalue, the R--R interval under consideration likely includes a far fieldR-wave and the Far Field R-Wave Criterion is met.

The processor keeps track of the number of R--R intervals out of apreceding series of intervals (e.g., 12 intervals) which likely containa far field R wave. This number (Far R Counter) is compared to athreshold value (Far R Threshold, e.g., 10) to determine whether it islikely that a heart rhythm which appears to have a high atrial rate isin fact the result of far field R-wave sensing.

FIG. 12 illustrates the basic operation of a device according to thepresent invention, in response to the occurrence of atrial andventricular events. In response to an atrial ventricular event at 100,the type of event is stored, and also a number of counts and valuesreferred to above are updated. In particular, in response to an atrialor ventricular event, the processor stores information as to the Pcount, i.e. the number of atrial events received since the lastventricular event, and an R count, i.e. the count of the number ofventricular events received since the last atrial event, and R--R, R-P,P--P and P-R intervals, as appropriate. The processor maintains buffersin the RAM, in which the following information is stored: the 12 mostrecent P--P intervals are stored, the 12 most recent R--R intervals arestored, the 8 immediately preceding R-P intervals, the 8 most recent P-Rinterval values, and the times of occurrence of atrial and ventricularevents over the preceding 12 R--R intervals, employed in conjunctionwith the detection of far field R waves, as discussed above. Inaddition, the processor also maintains a memory buffer of the binindexes for the preceding 18 R--R intervals, as described above inconjunction with the computation of the RR Modesum value and a buffercontaining the number of RR intervals over the preceding sequence of aprogrammable number of R--R intervals, which have durations less thanFDI, as discussed above in conjunction with the detection criterionadapted from the Model 7219 PCD device.

At 102, the processor updates all timing based features associated withthe occurrence of atrial and ventricular events, including allcomputations necessary to update the buffers described above,computation of all timing based values associated with the Model 7219detection criteria described above, including updating of the value ofVTEC, VFEC, the onset and stability counters, as well as updating the RRModesum value as described above, computation of the median values ofthe 12 preceding stored R--R interval durations, computation of themedian value of the stored preceding 12 P--P intervals and R--Rintervals, as appropriate, and in the case of a ventricular event,updates the beat code for the R--R interval ending with the ventricularevent.

In addition to these functions, in response to the occurrence of aventricular event, the processor at 103 computes the correspondingpattern code, as described above, associated with the R--R intervalending with the ventricular event and at 104 updates the continuousrecognition machine counters, as described above and the otherdiagnostic criteria described below in conjunction with the variousrules. The processor now has stored in RAM all information necessary toapply the hierarchical set of rules used to identify the particular typeof rhythm under way.

At 105, 106, 107, the processor determines which of the variousavailable rules have all of their respective clauses satisfied. Asdiscussed above, one, more than one, or no rules may have their causesall satisfied. If more than one rule is true or "fires", the rule ofhighest priority is selected at 108, leading to a rhythm classificationcorresponding to that rule at 109. In response to the classification ofthe rhythm, the device delivers therapy or prevents delivery of therapy,depending upon the rhythm identified. In the absence of any rules beingidentified, the device withholds anti-tachycardia therapy. If the deviceis programmed to provide bradycardia backup pacing, it continues to doso. If not, the device simply continues to monitor the rhythm of theheart, until one or more rules fire.

In the context of the specific embodiment disclosed herein, severalpossible rhythm classifications are provided by the rule set. Theseinclude ventricular fibrillation, ventricular tachycardia, simultaneousventricular and supraventricular tachycardia, simultaneous ventricularfibrillation and supraventricular tachycardia, atrial fibrillation orflutter, sinus tachycardia, AV nodal re-entrant tachycardia, normalsinus rhythm or "unclassified" rhythms, when no rules are "firing".

In conjunction with the present invention, 12 separate rules areemployed to identify the various rhythm types listed above. These rulesare in order of priority.

1. VF+SVT Rule

2. VT+SVT Rule

3. A Flutter Rule

4. A Fibrillation Rule

5. ST Rule

6. AVNRT Rule

7. NSR Rule

8. VT* Rule

9. VF Rule-7219

10. VT Rule-7219

11. Sustained AF Rule

12. Sustained AT Rule

Of the above rules, the A Flutter Rule, the A Fibrillation Rule, the STRule, the AVNRT Rule and the NSR Rule all prevent delivery ofventricular anti-tachyarrhythmia therapies. The VF+SVT rule, the VT+SVTrule, the VT* Rule, the VF Rule-7219 and the VT Rule-7219 all triggerdelivery of ventricular anti-tachyarrhythmia therapies. The Sustained AFRule and the Sustained AT Rule trigger delivery of atrialanti-arrhythmia therapies. As such, the hierarchical structure of therule base is such that the five lowest priority rules are provided fortriggering therapy, superseded by five intermediate priority rules forinhibiting delivery of anti-tachyarrhythmia therapy, which in turn aresuperseded by two high priority rules, triggering delivery ofanti-tachycardia therapy. This hierarchical rule structure is believedto be unique in the context of automated devices for triggering deliveryof anti-tachycardia therapies.

FIG. 13 illustrates the prioritization of the various rules, in the formof a flowchart. In response to occurrence of an R-wave at 600, each ruleis examined by the processor, in order of the priority listed aboveuntil one is met. If the first rule met is the VF+SVT Rule or VT+SVTRule at 602 or 604, VF therapy or VT therapy is delivered at 628 or 630,and delivery of atrial anti-arrhythmia therapies is prevented. If one ofthe rules which prevents treatment of ventricular tachyarrhythmias ismet at 606, 608, 610, 612 or 614, the processor examines whether theSustained AF Rule or Sustained AT Rule is the first rule met at 622 and624. If one of these rules is met, AF therapy or AT therapy is deliveredat 632 or 634. If no rules preempting ventricular therapies are met theprocessor examines whether the rules at 616, 618 or 620 are met, and ifso triggers delivery of VF or VT therapy at 628 or 630, preventingdelivery of AF or AT therapy. Similarly, if no rules preventing ortriggering ventricular anti-tachyarrhythmia therapy are met, theprocessor determines whether the Sustained AF Rule or the Sustained ATRule is the first rule met at 622 and 624 and if so triggers delivery ofthe appropriate therapy at 628 or 630. The specific rules and theirindividual clauses are described in detail below, illustrating theinterrelation of the various timing based and pattern based criteriadescribed above.

1. VF+SVT Rule

The VF+SVT Rule is the highest priority rule employed by the device, anddetects the simultaneous presence of VF and SVT. If it is met, ittriggers delivery of the next scheduled ventricular fibrillationtherapy, typically a high voltage defibrillation pulse. This rule hasfive clauses and is set true, or "fires" when all five clauses aresatisfied. The first clause requires that ventricular fibrillationdetection is programmed on and that any of rules 3-7 for preventingdelivery of ventricular anti-tachyarrhythmia therapies has also beenprogrammed on and that VFEC is greater or equal to VFNID, as discussedin conjunction with the VF detection criteria employed with the Model7219 discussed above. The second clause requires that the median valuefor the preceding 12 R--R intervals (RR median) is less than a presetminimum cycle length. This minimum cycle length may be VTDI, if VTdetection is programmed on or may be VFDI, if VT detection is programmedoff, or may be an interval separately programmable by the physician, ordefined as a fixed value within the device. The third clause requiresthat the median value for the preceding 12 R--R intervals is greaterthan a preset SVT Minimum Cycle Length. This SVT Minimum Cycle Lengthmust be less than VTDI, if VT detection is programmed on and must begreater than VFDI, if VT detection is programmed off and may be aninterval separately programmable by the physician in conjunction withprogramming of VTDI or VFDI.

The fourth clause employs an AF* Evidence Counter Criterion whichsupports or refutes the presence of atrial fibrillation using an up-downcounting algorithm performed by the processor, which increments ordecrements an AF* Evidence Counter based on atrial and ventricularpattern information. The AF* Evidence Counter Criterion will be met whenthe AF* Evidence Counter is greater than or equal to a predefined AF*Score Threshold, e.g. 6. Once the AF/AT Evidence Counter Criterion ismet, it will remain satisfied as long as the AF* Evidence Counter isgreater than or equal to a predefined AF* Score Hysteresis Threshold,e.g. 5. The fourth clause continues to be met as long as the AF* CounterCriterion continues to be met.

The AF* Evidence Counter is incremented and decremented as follows. Ifthe number of atrial events or P count in the current R--R interval is 1and the current beat code is the same as the previous beat code, the AF*Evidence Counter is decremented by 1, down to a minimum of 0. If thenumber of atrial events is 1 but if the beat codes are different the AF*Evidence Counter remains unchanged. If the number of atrial events inthe current R--R interval is greater than 2, then the AF* EvidenceCounter is incremented by 1, up to an AF* Score Maximum value, e.g. 10.If the number of atrial events in the current R--R interval is 2 and thecurrent beat code and the previous beat code are the same and the FarField R-Wave criterion discussed above is met for the preceding RRinterval, the AF* Evidence count remains unchanged. Otherwise the AF*Evidence Counter is incremented by 1, up to the AF* Maximum Score value.

The fifth and final clause of the rule employs an AV Dissociation CountCriterion implemented by the processor, which defines an AV DissociationCount, which is the number of a preceding series of R--R intervals, e.g.8 R--R intervals, which meet an AV Dissociation Criterion. The AVDissociation Criterion is met if there are no paced or sensed atrialevents in the current R--R interval or the absolute value of thedifference between the current P-R interval and the average of theprevious 8 P-R intervals is greater than 40 ms. The AV DissociationCount Criterion is met when the AV Dissociation Count is greater than orequal to a defined AV Dissociation Count Threshold, e.g. 4. When the A VDissociation Count Criterion is met, the fifth clause is satisfied.

If all of these clauses are satisfied, the rule is set true and "fires"triggering delivery of the next scheduled ventricular fibrillationtherapy. Firing of the VF+SVT rule supersedes firing of any other rules

2. VT+SVT Rule

The second highest priority rule is intended to identify thesimultaneous occurrence of ventricular tachycardia and supraventriculartachycardia. This rule contains six clauses, all of which must besatisfied in order for the rule to be set true or "fire". The firstclause requires that ventricular tachycardia detection be enabled, andthat the value of VTEC be greater than or equal to VTNID (as discussedabove in conjunction with the Model 7219 detection criteria). The secondclause requires that the AF* Evidence Counter Criterion as discussedabove is met. The third clause requires that the AV Dissociation CountCriterion discussed above is met. The fourth clause requires that the RRmedian is less than VTDI. The fifth clause requires that the RR medianis greater than the SVT Minimum Cycle Length discussed above. The sixthand final clause requires that the RR Modesum as described above iseither unknown or greater than a defined VT Plus RR Modesum Threshold,e.g. 0.75 of the preceding 12-18 R--R intervals.

If all of these clauses are satisfied, the rule is set true and "fires"triggering delivery of the next scheduled ventricular tachycardiatherapy. Firing of the VT+SVT rule supersedes firing of any other rules,with the exception of the VF+SVT rule, described above.

SVT Rejection Rules

The SVT rejection rules 3-7 cannot be applied if unless VT detection isProgrammed on, there have been at least enough intervals sinceinitialization of detection to fill the RR buffer, e.g. 12, and the RRmedian is greater than the SVT Minimum Cycle Length. The rules also havethe following sets of additional clauses.

3. A Flutter Rule

Due to the importance of distinguishing rapid ventricular rhythms due toatrial fibrillation or flutter from tachycardias of ventricular origin,two separate rules are provided for identifying the likely occurrence ofatrial fibrillation or flutter (or other atrial tachycardia). The firstof these two rules has two clauses which must be satisfied in order forthe rule to be met. The first clause requires that the value of CRMAL isgreater than or equal to its corresponding recognition threshold, e.g.6. The second clause requires that the Far Field R-Wave Count Criterionis met. The Far Field R-Wave Count Criterion is met when the Far FieldR-Wave Count is less than a defined Far Field R-Wave Count Threshold,e.g. 10 of the preceding 12 R--R intervals. If both clauses are met, therule is set true or "fires". If this is the highest priority firingrule, delivery of ventricular anti-tachyarrhythmia therapy is preventedeven if lower priority ventricular tachycardia or ventricularfibrillation rules are met while the rule is firing.

The A Flutter Rule is a "sticky" rule, meaning that when met, it remainsmet unless its clauses remain unsatisfied over a sequence of RRintervals. The processor accomplishes this result by setting anassociated AF Rejection Sticky Count to a predefined value, e.g. 6whenever the rule is met. For each R--R interval for which either thefirst or second clause is not met, the Sticky Count is decremented by 1to a minimum of 0. The rule continues to fire as long as the StickyCount remains above 0.

4. A Fibrillation Rule

The second rule directed toward detection of the occurrence of atrialfibrillation or flutter (or other atrial tachycardia) has four clauseswhich must be met. The first clause requires that the Far Field R-WaveCount Criterion, discussed above, is met. The second clause requiresthat the median value of the P--P interval, over the preceding 12 R--Rintervals be known, and that it be less than a preset value, e.g. 87.5%of the corresponding RR median value, over the preceding 12 intervals.The third clause requires that AF* Evidence Counter Criterion issatisfied, as discussed above. The fourth clause requires that the RRModesum is less than or equal to a defined AF Modesum Threshold, e.g.0.5 of the previous 12-18 intervals. If all four clauses of the rule aresatisfied, the rule is set true or "fires". If this rule is the highestfiring priority rule, delivery of ventricular anti-tachyarrhythmiatherapies is prevented.

The A Fibrillation Rejection Rule is a "sticky" rule, meaning that whenmet, it remains met unless its clauses remain unsatisfied over asequence of RR intervals. The processor accomplishes this result bysetting an associated AFib Rejection Sticky Count to a predefined value,e.g. 6 whenever the rule is met. For each R--R interval for which any ofthe four clauses are not met, the Sticky Count is decremented by 1 to aminimum of 0. The rule continues to fire as long as the Sticky Countremains above 0. The Sticky Count is reset to 0 on initialization ofdetection and whenever a higher priority SVT rejection rule issatisfied.

5. ST Rule

This rule is directed toward recognition of sinus tachycardia, andincludes three clauses, of which either the first clause or the secondand third clauses must be met in order for the rule to fire. The clauserequires that CRMedST exceed its corresponding recognition threshold,e.g., 6. If this clause is satisfied, the rule fires. The second clauserequires that the Far Field Counter Criterion discussed above be met.The third clause requires that the CRMedSTFR exceed its correspondingrecognition threshold, e.g. 6. If the second and third clauses aresatisfied, the rule fires. If the ST Rule is the highest priority rulefiring, delivery of anti-tachycardia therapies is prevented.

The ST rule is a "sticky" rule, meaning that when met, it remains metunless its clauses remain unsatisfied over a sequence of RR intervals.The processor accomplishes this result by setting an associated SinusRejection Sticky Count to a predefined value, e.g. 6 whenever the ruleis met. For each R--R interval for which either the first clause is notmet or for which one or both of the second and third clauses is not met,the Sticky Count is decremented by 1 to a minimum of 0. The rulecontinues to fire as long as the Sticky Count remains above 0. TheSticky Count is reset to 0 on initialization of detection and whenever ahigher priority SVT rejection rule is satisfied.

6. AVNRT Rule

This rule is directed toward detection of AV nodal re-entranttachycardia. The rule includes two clauses, each of which must besatisfied in order for the rule to fire. The first clause requires thatCRMAVNRT exceed its corresponding threshold value, e.g. 6. The secondclause requires that RR Modesum is greater than or equal to a definedAVNRT Modesum Threshold, e.g. 0.25 of the preceding 12-18 R--Rintervals. If both clauses are satisfied, the rule is set true or"fires". If it is the highest priority firing rule, it prevents deliveryof ventricular anti-tachycardia therapies.

The AVNRT Rule is a "sticky" rule, meaning that when met, it remains metunless its clauses remain unsatisfied over a sequence of RR intervals.The processor accomplishes this result by setting an associated AVNRTSticky Count to a predefined value, e.g. 6 whenever the rule is met. Foreach R--R interval for which either the first or second clause is notmet, the Sticky Count is decremented by 1 to a minimum of 0. The rulecontinues to fire as long as the Sticky Count remains above 0. TheSticky Count is reset to 0 on initialization of detection and whenever ahigher priority SVT rejection rule is satisfied.

7. NSR Rule

This rule is directed toward detection of a normal sinus rhythm, andincludes three clauses of which either the first clause or the secondand third clauses must be met in order for the rule to fire. The clauserequires that CRMedST exceed its corresponding recognition threshold,e.g., 6. If this clause is satisfied, the rule fires. The second clauserequires that the Far Field Counter Criterion discussed above be met.The third clause requires that the CRMedSTFR exceed its correspondingrecognition threshold, e.g. 6. If the second and third clauses aresatisfied, the rule fires. If the ST Rule is the highest priority rulefiring, delivery of anti-tachycardia therapies is prevented.

The ST rule is a "sticky" rule, meaning that when met, it remains metunless its clauses remain unsatisfied over a sequence of RR intervals.The processor accomplishes this result by setting an associated SinusRejection Sticky Count to a predefined value, e.g. 6 whenever the ruleis met. For each R--R interval for which either the first clause is notmet or for which one or both of the second and third clauses is not met,the Sticky Count is decremented by 1 to a minimum of 0. The rulecontinues to fire as long as the Sticky Count remains above 0. TheSticky Count is reset to 0 on initialization of detection and whenever ahigher priority SVT rejection rule is satisfied.

The next three rules are ventricular fibrillation and tachycardiadetection rules which trigger delivery of ventricularanti-tachyarrhythmia therapies.

8. VT* Rule

The VT* Rule discriminates fast VT with regular cycle lengths from VF.This rule has three clauses which must be satisfied, in order for therule to be set true. The first clause simply requires that VF detectionand VT detection are enabled and that the model 7219 VF detectioncriteria are met, i.e. VFEC is greater than or equal to VFNID. Thesecond clause requires that RR median is greater than or equal to theFast VT Minimum Cycle length, discussed above. The third clause requiresthat the VT* RR Modesum Criterion is satisfied. The VT* RR ModesumCriterion is satisfied when RR Modesum is either unknown or greater thanor equal to the a defined Fast VT Modesum Threshold, e.g. 0.75 of thepreceding 12-18 R--R intervals.

9. VF Rule-7219

This rule corresponds to the detection criteria for ventricularfibrillation as set forth above in conjunction with the description ofthe Model 7219 device. If VF is detected using these criteria, the ruleis set true and "fires" if it is the highest firing rule, it triggersdelivery of the next scheduled ventricular fibrillation therapy.

10. VT Rule-7219

This rule simply restates all the ventricular tachycardia detectioncriteria provided in the Model 7219 device, as discussed above, withdetection of fast ventricular tachycardia disabled. In the event thatthis rule is the highest firing rule, it triggers delivery of the nextscheduled VT therapy.

In conjunction with above rule set, it should be understood that in theevent that a rule triggering delivery of a ventricular tachycardiatherapy fires, subsequent firing of a rule indicative of the occurrenceof a supraventricular tachycardia cannot occur, as the pattern grammar,and/or other timing criteria cannot possibly be met after initiation ofanti-tachycardia therapy. However, it is certainly possible for a ruleindicating the occurrence of a ventricular tachyarrhythmia to fire whilea rule indicative of the occurrence of a supraventricular tachycardia isfiring. In such case, the highest priority firing rule dominates. Itshould also be understood that rules 1-8 above are "sticky" rules,meaning that once a rule has fired, it will continue to fire until oneor more clauses of the rule are not satisfied for a sequence of apredetermined number of R--R intervals. A nominal value for thispredetermined number of R--R intervals is three, however, it isenvisioned that the parameter may be programmable by the physician. Thisfeature is intended to prevent a temporary violation of one of theclauses of a rule, for one or two beats, to override the firing of therule. This is particularly important in the context of the rulesintended to detect the likely occurrence of atrial tachycardias, where aone or two beat failure of the rule to be met could well result in thedelivery of a ventricular anti-tachycardia therapy, in conjunction withthe firing of a lower priority VT or VF detection rule, resulting ininappropriate delivery of ventricular anti-tachycardia therapy.

11 and 12. Sustained AF and Sustained AT rules

In conjunction with a preferred embodiment of the invention, rules fortriggering delivery of anti-arrhythmia therapies in response to detectedsustained atrial fibrillation and/or sustained atrial tachycardia arealso included. These rules are interrelated in operation and so arediscussed together. Both rules cannot be met simultaneously. Inconjunction with these rules, an additional set of defined parameters isemployed. The additional parameters include an atrial fibrillationdetection interval (AFDI), which may be for example 150-300 ms, anatrial tachycardia detection interval (ATDI), which may be, for example,up to 450 ms, but in any case greater than AFDI, and a minimum atrialtachycardia interval (AT Minimum Interval), which may be for example100-300 ms, but in any case less than ATDI. These parameters, andothers, are used by the processor in conjunction with an additional setof diagnostic criteria, as set forth below.

A first criterion, associated with detection of atrial fibrillation isthe AF Rate Zone Criterion. This criterion in turn is based upon twomeasured characteristics of the heart rhythm, including the medianinterval separating preceding atrial depolarizations (PP Median) and theregularity of the atrial cycle length (Cycle Length Regularity CounterCriterion). On each ventricular event, the buffer containing theprevious 12 atrial cycle lengths will be examined to determine themedian P--P interval and to determine regularity. The atrial cyclelengths are classified as being regular on a given ventricular event ifthe difference between the second to longest and the second to shortestatrial cycle length in the buffer is less than or equal to the PP Mediandivided by 4. The Atrial Cycle Length Regularity criterion will besatisfied if the atrial cycle length regularity condition is met on 6 ofthe most recent 8 ventricular events. The AF Rate Zone Criterion issatisfied when the PP Median is less than the programmed AFDI ifSustained AT detection is programmed off. If Sustained AT detection isprogrammed on then the AT Rate zone Criterion is met when the PP Medianis less than the programmed AFDI, and either the PP Median is less thanthe programmed AT Minimum Interval or the Cycle Length RegularityCounter Criterion is not satisfied.

A second criterion, associated with detection of atrial tachycardia isthe AT Rate Zone Criterion. The AT Rate Zone criterion uses the PPMedian and the Atrial Cycle Length Regularity Criterion to identify ATand to discriminate it from AF. The AT Rate Zone Criterion is satisfiedwhen the PP Median is less than the programmed ATDI and greater than orequal to the programmed AFDI, or when the PP Median is less than AFDIbut greater than or equal to the programmed AT Minimum Interval and theAtrial Cycle Length Regularity Counter Criterion is satisfied.

A third criterion, associated with detection of both AF and AT is theAF/AT Evidence Counter Criterion which supports or refutes the presenceof an atrial arrhythmia using an up-down counting algorithm whichincrements or decrements an AF/AT Evidence Count based on atrial andventricular pattern information. The AF/AT Evidence Counter Criterionwill be met when the AF/AT Evidence count is greater than or equal to apredefined AF/AT Score Threshold, e.g. 32. Once the AF/AT EvidenceCounter criterion is met, it will remain satisfied as long as the AF/AFEvidence count is greater than or equal to a predefined AF/AT ScoreHysteresis Threshold, e.g. 27.

In conjunction with the AF/AT evidence Counter Criterion, severaladditional characteristics of the heart's rhythm are monitored. Oneadditional monitored characteristic is the Sinus Rhythm CounterCriterion, which identifies regular sinus rhythm with 1:1 conduction ora paced rhythm. The Sinus Rhythm Counter (SR Counter) is be affected bythe beat code as defined above, as follows. If the beat code is 0, 1 isadded to the SR Counter up to a maximum of 255. Otherwise the SR Counteris set to 0. The Sinus Rhythm Counter Criterion will be satisfied whenthe SR Counter is greater than or equal to a predefined the AF ResetCount Threshold, e.g. 5. The Sinus Rhythm Counter Criterion is suspendedwhile a therapy operation is in progress. The SR Counter is set to zerowhen detection is initialized.

Also employed in conjunction with the AT/AF Evidence counter is theSinus Rhythm with Far Field R-wave Criterion, which identifies sinusrhythm in the presence of far field R-waves. On each ventricular event aSinus Rhythm with Far Field R-wave Counter will be updated as follows.If the Far Field R-wave criterion discussed above is satisfied for thecurrent RR interval and the current ventricular beat code is 9, 4 or 6,1 is added to the Sinus Rhythm with Far Field R-wave Counter up to amaximum of 255.

Otherwise the Sinus Rhythm with Far Field R-wave Counter is reset to 0.The Sinus Rhythm with Far Field R-wave Counter Criterion is satisfiedwhen the Sinus Rhythm with Far Field R-wave counter is greater than orequal to the AF Reset Count Threshold. The Sinus Rhythm with Far FieldR-wave Counter Criterion is suspended while a therapy operation is inprogress. The Sinus Rhythm with Far Field R-wave Counter is initializedto 0 when detection is initialized.

On each ventricular event the AF/AT Evidence Counter will be updated asfollows. If the Sinus Rhythm Count Criterion is satisfied or the SinusRhythm with Far Field R-wave Count Criterion specified is satisfied, theAF/AT Evidence Counter is reset to 0.

If neither the Sinus Rhythm Count Criterion is satisfied or the SinusRhythm with Far Field R-wave Count Criterion is satisfied, and if the Pcount (number of atrial events in the RR interval, discussed above inconjunction with Beat Codes) is less than or equal to 1 and the AF/ATEvidence Counter was incremented on the last ventricular event, 1 isadded to the AF/AT Evidence Counter up to a predefined the AF ScoreMaximum Value, e.g. 47.

If neither the Sinus Rhythm Count Criterion is satisfied or the SinusRhythm with Far Field R-wave Count Criterion is satisfied, and the Pcount is equal to 2 and the Far Field R-wave Criterion discussed aboveis met for the current ventricular event and the AF/AT Evidence Counterwas incremented on the last ventricular event, 1 is added to the AF/ATEvidence Counter up to a predefined the AF Score Maximum Value.

If neither the Sinus Rhythm Count Criterion is satisfied or the SinusRhythm with Far Field R-wave Count Criterion specified is satisfied, andthe P count is equal to 2 and the Far Field R-wave criterion discussedabove is not met for the current ventricular event, 1 is added to theAF/AT Evidence Counter up to the AF Score Maximum Value.

If neither the Sinus Rhythm Count Criterion is satisfied or the SinusRhythm with Far Field R-wave Count Criterion specified is satisfied, andthe P count is more than 2, 1 is added to the AF/AT Evidence Counter upto the AF Score Maximum Value.

If none of the above conditions applies, 1 is subtracted from the AF/ATEvidence Counter down to a minimum value of 0.

Detection of sustained atrial fibrillation or sustained atrialtachycardia begins with preliminary detection of these rhythms.Preliminary detection of AF occurs when the AF/AT Detection EvidenceCount Criterion and the AF Rate Zone Criterion discussed above are bothmet. Preliminary detection of AF will result in the start of thesustained AF/AT duration timer, described in more detail below.Preliminary detection of AT occurs when the AF/AT Detection EvidenceCount Criterion and the AT Rate Zone Criterion discussed above are bothmet. Preliminary detection of AT similarly results in the start of thesustained AF/AT duration timer. Preliminary Detection of AT or AF willbe possible only if VT or VF is not detected by the device using therules described above. AT and AF detection will be suspended if adetected VT or VF episode is in progress.

The sustained AF/AT duration timer is initiated on preliminary detectionof AF or AT and continues to time until termination of atrialtachyarrhythmia is detected. The sustained duration timer continues totime through delivery of anti-atrial tachyarrhythmia therapies. Thesustained AF/AT duration timer is used in conjunction with one or moredefined minimum required durations, e.g. 1-1440 minutes, programmable bythe physician, associated with either the arrhythmia determined to beunderway and/or the type of therapy next scheduled for delivery. forexample, the minimum sustained duration for a scheduled pacing pulselevel therapy would typically be less than for a high voltage therapydelivered in response to detection of AF. No therapy for a detectedarrhythmia, i.e. AT or AF can be delivered following delivery of atherapy for the same arrhythmia which has a longer defined minimumsustained duration. The type of arrhythmia underway, followingactivation of the sustained AF/AT duration timer may be AT, AF, orundefined, is determined according to the following method. The criteriafor preliminary detection of AF and AT discussed above are continuallyapplied following initial detection. The criterion (AF or AT) presentlymet is the arrhythmia determined to be present. A failure to meet theAF/AT Evidence Counter Criterion or a failure to meet either of the ATand AF Rate Zone Criteria results in the arrhythmia being designated asunclassified. If the arrhythmia is classified as AT or AF, and if theapplicable minimum required duration associated with the arrhythmiadetermined to be present and/or the next scheduled therapy has beenexceeded, the next scheduled therapy is delivered, to any associatedadditional preconditions for therapy discussed below also being met. Notherapy can be delivered while the arrhythmia is unclassified.

FIG. 14 illustrates the interrelation of the sustained AF/AT durationtimer, the AF/AT evidence counter and the AF and AT Rate Zone Criteriain detecting sustained AF or AT and triggering delivery of anti-atrialarrhythmia therapy. At 500, The AF/AT Evidence counter begins to beincremented as described above. Concurrently the PP Median, AF Rate ZoneCriteria and AT rate Zone Criteria are monitored. Preliminary detectionof AT occurs, when the AF/AT Evidence Count reaches the required minimumduration at 502, with initial classification of the arrhythmia as AToccurring at 504, as the AT Rate Zone Criterion is also concurrentlymet. At 506, The arrhythmia is reclassified to AF, due to the AF RateZone Criterion being met. Subsequent changes in classification occur,with the arrhythmia being unclassified at 510 in response to the AF/ATEvidence Counter Criterion failing to be met at 508. When the AF/ATEvidence Counter Criterion is again met at 512, the arrhythmia isclassified as AT due to the AT Rate Zone criterion being met. Asillustrated, a Hysteresis AF/AT Evidence count Threshold is alsodefined.

In FIG. 14, a single defined minimum sustained duration is illustratedat 522. This would be the case if the minimum sustained duration isdefined only by the next scheduled therapy type (e.g. high voltage shockvs. low energy, pacing pulse level therapies. However, if desired,different minimum sustained durations may also be defined for differentarrhythmia types, as discussed above. At 516, the applicable minimumsustained duration is reached, concurrent with the arrhythmia beingclassified as AF, triggering delivery of the next scheduled AF therapy.Following delivery of the therapy, the AF/AT Evidence Counter is resetat 518, with redetection of AF occurring at 520, when the AF EvidenceCounter again reaches the threshold.

As discussed above, the Sustained AF/AT Duration Timer continues to timeuntil termination of atrial tachyarrhythmia is detected. Satisfaction ofthe AF/AT Episode Termination criterion will defines the end of asustained AF/AT Episode, resets the Sustained AF/AT Duration Timer, andrestores preliminary AF/AT detection conditions. The AF/AT EpisodeTermination Criterion is satisfied when either the Sinus Rhythm CounterCriterion discussed above is satisfied, or the Sinus Rhythm With FarField R-wave Counter Criterion discussed above is satisfied, ordetection has resumed for a predetermined time period, e.g. threeminutes after being suspended (as discussed below) and the arrhythmiahas not been classified in that time period as AF or AT, or a VT episodeor VF episode is detected as discussed above.

All AF/AT detection is temporarily suspended when an atrialanti-tachyarrhythmia therapy is in progress. When detection is suspendedthe device will operate as follows. The arrhythmia classification willbe set to unclassified, but the device will continue to update theSustained AF/AT Duration Timer, if it is currently in operation.Similarly, the device will continue to look for AF/AT termination ofawhile the device is in the suspend detection state. When suspension ofdetection ends the device will initialize detection criteria other thanthe Sustained AF/AT Duration Timer, such that a full detection (orre-detection) sequence will be required to classify the rhythm or detectepisode termination. Temporary suspension of detection will end whendelivery of therapy is terminated.

Optionally, the device may be programmable to also suspend AF/ATdetection for 16 ventricular intervals following therapy delivery.During this period the effective AFDI and ATDI will be set to zero (i.e.the AF and AT detection zones will be disabled). This feature isbelieved particularly desirable in conjunction with the High frequencystimulation therapies disclosed in the Mehra and Duffin patents citedabove, to provide additional time needed for termination of atrialtachyarrhythmias treated with such therapy.

In preferred embodiments of the invention, additional prerequisitecriteria for delivery of anti-atrial tachyarrhythmia therapies may beincluded. For example, AF/AT therapy may be disabled due to ventriculararrhythmia detection following AF/AT Therapy. Confirmation of AF/AT and/or expiration of a minimum delay since the delivery of a previoustherapy may be prerequisites and a specified time of day may beprerequisites to delivery of AF/AT therapy. Expiration of a maximumsustained AF/AT duration and/or a predefined number of therapies havingbeen delivered in a preceding time period may prevent delivery of AT/AFtherapy. These additional criteria are discussed below.

The detection of VT or VF following the delivery of an AF/AT therapyprior to-either re-detection of AF/AT or AF/AT episode termination canoptionally cause the device to disable all subsequent AF/AT therapyuntil the condition has been cleared by the physician. An AF/AT therapydisabled flag in this case would be set by the microprocessor would beavailable and may be cleared via telemetry, by the physician, ifdesired. This feature will prevent further AT/AF therapy when it hasbeen closely associated with a detected episode of VT or VF. AF/ATdetection may continue following termination of the VT or VF episode,however, no AF/AT therapies would be delivered.

Optionally, the device may retain a running count of the number highvoltage AF/AT therapies delivered over the preceding 24 hours. An AtrialHigh Voltage Therapies per 24 Hour Cycle Criterion would be satisfied ifthe atrial high voltage therapy count is less than a programmed MaximumNumber of Atrial High Voltage Therapies per 24 Hour Cycle. Satisfactionof the Atrial High Voltage Therapies per 24 Hour Cycle Criterion may berequired as prerequisite to delivery of high voltage AT/AF therapies.

As discussed in U.S. patent application Ser. No. 08/434,899, by Bardy,for an "Atrial Defibrillator and Method of Use", filed May 3, 1995 andincorporated herein by reference in its entirety, it may also bedesirable to limit delivery of high voltage therapies to a defined timeperiod when the patient is likely to be asleep. A Time of Day AtrialHigh Voltage Therapy Criterion can prevent automatic atrialdefibrillation therapy from being delivered outside of a programmed timewindow.

If a sustained episode of AF or AT persists for long enough, thephysician may wish to prevent further attempts of the device toterminate the arrhythmia. In such case, A Time to Stop Therapy Criterionmay be employed to disable AF and AT therapy when the Sustained AF/ATDuration Timer exceeds a programmed Time to Stop Therapy, e.g. more than48 hours.

Confirmation of that a sinus rhythm has not resumed may also be requiredas a prerequisite to delivery of AF/AT therapy. An AF/AT TherapyConfirmation Criterion will prevent the initiation of atrial therapywhen sinus rhythm has returned but AF/AT episode termination has not yetbeen detected. The AF/AT Therapy Confirmation Criterion may be satisfiedfor the current ventricular interval if either the number of atrialevents in the current ventricular interval is greater than two, or thenumber of atrial events in the current ventricular interval is two andthe atrial interval for both events is either less than the ATDI if ATdetection is ON or AFDI if AT detection is OFF.

A minimum interval between delivered therapies may also be aprerequisite to AF therapy. A Post Therapy AF Therapy Delay Criterionmay be employed to delay the initiation of AF therapy delivery of aprior AF therapy. This will allow nonsustained atrial fibrillationresulting from the therapy to spontaneously terminate before AF therapyintervention. It may also be used to create a delay between AFtherapies. The Post AF Therapy Delay may be, for example, 240 seconds.The Post Therapy AF Therapy Delay Criterion is satisfied if either no AFtherapies have been delivered in the current AF/AT episode, or he numberof seconds since the last therapy scan delivered with the post therapyAF therapy delay enabled is greater than the Post Therapy AF TherapyDelay, and satisfaction of this criterion may be a prerequisite todelivery of AF therapy.

In conjunction with commercial embodiments of devices according to thepresent invention, it is anticipated that selecting which of the variousavailable rules are to be activated in the device may prove anoverwhelming task for the physician. As such, it is proposed that VF,VT, AF and AT detection and treatment using rules 8, 9, 10, 11 and 12may be programmed only in specific combinations, such that if AF, AT orVT detection and therapies are enabled, then VF detection and therapiesmust also be enabled as a safeguard. Similarly, if AT detection andtherapies are enabled, then AF and VF detection and therapies must alsobe enabled.

With regard to rules 3-7, these rules may be programmed on or offindividually by the physician. However, simultaneous VF and SVTdetection and therapy using rule 1 are automatically enabled in responseto any of rules 3-7 being enabled along with VF detection and therapyusing rule 9. Similarly, simultaneous VT and SVT detection and therapyusing rule 2 is automatically enabled in response to any of rules 3-7being enabled along with VT detection and therapy using rule 8 or 10. Itshould also be noted that under this proposed approach to selecting setsof rules to be activated, that the highest priority rules 1 and 2, whichtrigger delivery of therapy are not enabled in the absence ofennoblement of one or more of intermediate priority rules 3-7, whichinhibit delivery of anti-tachycardia therapy. The reason for this isthat the higher priority rules 1-2 set forth stricter requirements fordetection of ventricular fibrillation and tachycardia than rules 8-10,and are thus unnecessary, in the absence of intermediate priority rules3-7, capable of overriding the VT and VF detection criteria defined bythese rules.

While the above rule set is described in terms of initial detection of atachyarrhythmia, such a prioritized rule system may also be employed inconjunction with redetection of a tachyarrhythmia or in detection of achange of type of ventricular tachyarrhythmia. However, due to thecomplexities of such a system, it is proposed that as a practicalmatter, the device may simply be programmed such that following deliveryof an initial tachycardia therapy, detection of termination of thearrhythmia and redetection of ventricular tachyarrhythmias be conformedto that employed in the Model 7219, for the sake of ease of use andsimplicity. In such an embodiment, delivery of an initial ventricularanti-tachyarrhythmia therapy will result in disablement of Rules 1-8until subsequent detection of termination of the detected ventriculartachyarrhythmia, following which Rules 1-8, as selected by thephysician, may be reactivated. Redetection of atrial tachyarrhythmias isdone using the criteria for preliminary detection, as described above inconjunction with rules 11 and 12.

While the AF/AT Evidence counter, the AF and AT Rate Zones and the AF/ATSustained Duration Timer are disclosed as useful in detecting atrialtachyarrhythmias, it should be understood that the basic framework forarrhythmia detection they provide may also be useful to detectventricular tachyarrhythmias. In particular, the basic functionalinterrelation of these elements of the device may be applicable in ananalogous fashion to distinguish between ventricular tachycardias and/ornodal tachycardias.

2. Operation of patient activator

FIG. 15 is a functional flow chart illustrating the operation of thepatient activator in conjunction with a request for therapy. In responseto the pressing of the activator button at 800, thepower/switching/battery monitoring circuit 120 powers up the activator,and the microprocessor defines two time intervals thereafter, includinginterval T-1, which may be 10 seconds, and interval T-3, which may be 60seconds. The microprocessor polls the battery monitoring circuit 120 at802 to determine whether adequate battery voltage is present, if not,the microprocessor provides a signal to circuitry 120 at 804 to shutdown the activator. If battery voltage is adequate, the microprocessortriggers the LED drivers 114 to flash both the green and amber LEDs for250 milliseconds, which indicates that the activator is operative. At808, the microprocessor modulates a programming data stream, which ispresented to antenna driver/switching circuit 124, for transmission tothe implanted device. The microprocessor then waits at 809 for a returnuplink signal from the implanted device, indicating that the patientactivation request has been received. On transmission of the downlink,the microprocessor also defines a time interval T-2, which for example,may be 250 milliseconds. At 810, the microprocessor determines whetherit has received a valid uplink from the implanted device. If thedownlink to the device was received, the corresponding uplink wouldstart within T-2 minus the duration of the uplink (e.g. 70 ms.)following the end of transmission of the downlink. If an uplink is notreceived or the uplink that is received cannot be adequately decoded,the device checks at 814 to determine whether T-1 has terminated. Ifnot, the device waits until the end of time interval T-2 at 812, andrepeats the downlink transmission, until either time period T-1 has beencompleted, or a good uplink has been received.

If time period T-1 expires prior to receiving a good uplink from theimplanted device, the microprocessor checks at 816 to see if the pushbutton is currently being pressed, indicating that the patient stilldesires delivery of therapy. If not, the microprocessor signals thepower/switching/monitoring circuit 120 to power down the device. If thebutton has not been released after expiration of time interval T-1, themicroprocessor checks at 818 to determine whether time interval T-3 hasexpired, which is taken as an indication that communication with theimplanted device simply is not possible, and the microprocessor simplyshuts down the activator at 820. If the button is being pressed, andtime interval T-3 has not been completed, the activator continues tosend downlink patient therapy requests every 250 milliseconds, untileither the button is released, or T-3 expires.

On receipt of a valid uplink at 810, the microprocessor defines twoadditional time periods including T-4, which may be 60 seconds since theuplink was received, and T-5, which may be 10 seconds since the uplinkwas received. At 822 the uplink is decoded, and the activator isnotified whether or not the patient's heart rhythm indicates thattherapy is appropriate at 824. If therapy is not appropriate, at 828 themicroprocessor triggers the speaker driver 110 to deliver an audiblesignal, for example, a sequence of three rising tones, and activates theLED driver circuitry 114 to flash the green indicator light on the caseof the activator, indicating to the patient that therapy is notwarranted and will not be delivered. If therapy is appropriate butcannot presently be delivered at 826, the microprocessor notifies thepatient at 830 by activating the speaker driver 110 to produce adifferent audio signal, for example, a falling two-tone sequence. Inaddition, the microprocessor activates the LED driver circuitry 114 tocause both the amber and the green lights to flash. In this case, thepatient knows that the therapy is warranted but will not be delivered.If, on the other hand, the implanted device determines that therequirements for the therapy are presently met, the microprocessornotifies the patient at 832 by a third, distinguishable audio signal,for example a steady, pulsing tone, in conjunction with activation ofthe amber LED by LED driver circuitry 114. The particular audio andvisual signals provided to the patient, are of course, arbitrary, andany set of similar warning signals which allows the patient to reliablydistinguish between the three states, i.e., no therapy, therapypresently unavailable, and therapy pending, are believed to be workablein the context of the present invention. On expiration of interval T-5,the LED and audio signals are terminated, and the microprocessor checksat 838 to determine whether the push button is released. On eitherexpiration of time interval T-4 at 836 or release of the push button at838, the activator is powered down.

3. Arrhythmia detection following patient request

Operation of the implantable device in response to a patient-activationrequest is as follows. The request for patient-activated therapy isreceived by telemetry receiver 330 by means of antenna 332. The decodedtelemetry signal is applied to multiplexer 220, and provided to themicroprocessor by means of address/data bus 218. The microprocessor inresponse to receiving a request for patient-activated therapy, attemptsto determine whether patient-activated therapy is appropriate, inaccordance with a subset of the arrhythmia detection criteria describedin detail above. In the particular embodiment disclosed herein, themicroprocessor determines whether a detected AF/AT episode is inprogress. As discussed above in conjunction with FIG. 14, an episode isconsidered to be in progress if the AF/AT evidence count previouslyreached its threshold value and termination has not been detected. Inaddition, the microprocessor checks to determine whether the AF/ATTherapy Confirmation criterion described above is satisfied and checksto determine that the Time to Stop Therapy criterion described above isnot satisfied and that AF/AT therapy has not been disabled due todetection of ventricular tachyarrhythmia following delivery of an atrialtherapy, also as described above. The microprocessor also checks todetermine whether patient activated therapy has been enabled byprogramming and if the patient activated therapy is a high voltagetherapy such as cardioversion or defibrillation the processor checks tosee whether the high voltage charging circuit has been disabled forexample due to excessive charging times as in currently marketedimplantable defibrillators. If the rhythm classification is either AF orAT, AF/AT therapy has not been disabled, the AF/AT Therapy Confirmationcriterion is satisfied and the Time to Stop Therapy criterion is notsatisfied, patient activated therapy is enabled and the charging circuitis not disabled, the patient-activated therapy is scheduled and themicroprocessor triggers an uplink transmission to the patient activatorindicating that therapy is pending. If the programmed patient initiatedtherapy is atrial defibrillation or cardioversion, an additionalprerequisite to scheduling the therapy is that either at least two ofthe preceding twelve ventricular intervals are greater than theventricular refractory interval (indicating that synchronization islikely possible) or there have been fewer than twelve intervals sincethe detection functions were last initialized.

Once the therapy is scheduled, it will be delivered at such time as therhythm classification is either AT or AF, as discussed in conjunctionwith FIG. 14, provided that the AF/AT Confirmation criterion remainssatisfied and the Time to Stop Therapy criterion remains unsatisfied,and provided that the pending therapy is not canceled, as discussedbelow. It should be noted that the criteria for scheduling the therapyas pending do not require the rhythm to be classified as AF or AT, so itis possible to schedule a therapy without meeting the criteria fordelivering it. When the patients rhythm thereafter meets the criteriafor delivery, the therapy is initiated.

A pending therapy may be canceled before delivery if the need fortherapy terminates or the device becomes incapable of delivering thetherapy. For example, therapy may be canceled if termination of theAF/AT episode is detected, if AF/AT therapy is disabled due to detectionof ventricular tachyarrhythmia following a previous atrialanti-arrhythmia therapy, or if the Time to Stop Therapy criterion ismet. If the scheduled therapy is defibrillation or cardioversion, thetherapy may also be canceled upon a determination that the time tocharge the high voltage output capacitors exceeds a preset duration, asis conventional in commercially marketed implantable defibrillators.Finally, therapy may be canceled if the criteria for delivery of thetherapy are not satisfied within a predefined time period, e.g. oneminute, following scheduling of the therapy. By this mechanism, failureof the rhythm to continuously meet the criteria for delivery will betolerated for a period of time, but not indefinitely.

The methodology employed by the implanted device and patient activatorin combination thus provides a mechanism for delivering patientrequested therapy even if, the implanted device is unable to classifythe rhythm at the time the button on the activator is pushed, so long asthe criteria for delivery are met within a reasonable time thereafter.The criteria for triggering a patient activated therapy are based uponthe criteria for triggering a device initiated therapy, but are lessstringent, as they are a subset of the required criteria for deviceinitiated therapy. The criteria for delivery are thus likely to be metquickly if therapy is appropriate, facilitating the prompt, appropriatedelivery of therapy in response to a patients request.

The above disclosure sets forth a device in which sensed events in theatrium and ventricle are used to control delivery of electrical therapyto treat tachyarrhythmias. However, the basic methodology of the presentinvention is believed equally applicable to devices which deliver othertypes of therapies, such as automatic delivery of anti-arrhythmic drugs,as disclosed in U.S. Pat. No. 4,146,029, issued to Ellinwood, U.S. Pat.No. 5,527,344, issued to Arzbaecher et al., and U.S. Pat. No. 5,087,243,issued to Avitall, all of which are incorporated herein by reference intheir entireties. Identification of the origin of the arrhythmia anddelivery or withholding of therapy in response to a patient requestwould be equally valuable in such devices. Furthermore, while it seemslikely that commercial embodiments of devices according to the presentinvention will require the use of a microprocessor in order to performthe calculations and analysis steps required for arrhythmia detection,it is within the realm of possibility that some or all of the detectionand control functions provided by the microprocessor might instead beprovided by means of a full custom, integrated circuit, particularly acircuit in which a state counter is employed instead of stored software,in order to control sequential operation of the digital circuitry, alongthe general lines of the circuits disclosed in U.S. Pat. No. 5,088,488,issued to Markowitz et al. and U.S. Pat. No. 5,052,388, issued to Sivulaet al., both of which are incorporated herein by reference in theirentireties.

The above disclosed embodiment employs a particular detectionmethodology which is unique to the products of the assignee. However, itis believed that the basic mechanism of the present invention could alsobe beneficially incorporated into anti-arrhythmia devices usingdifferent detection methods, such as disclosed in any of the variouspatents cited above directed to implantable anti-arrhythmia devices, byemploying a subset of the criteria or less stringent individual criteriarequired for device initiated therapy as a confirmation of a patient.For example, in a device which relies on parameters such as rate,sustained high rate, rate regularity, and/or sudden onset as inpresently marketed implantable defibrillators, fewer of these parameterscould be employed for patient-triggered therapy than for deviceinitiated therapies. Alternatively, or in addition, the individualcriteria may be altered, for example by a lowered required rate or ashortened sustained high rate period, as may be appropriate. Inaddition, the provision of a time interval following patient activationduring which the criteria for therapy delivery must be met is believedvaluable even in the context of patient activators which requireverification of arrhythmia using the same criteria as for deviceinitiated therapy. Thus, the above description should be consideredexemplary, rather than limiting, with regard to the interpretation ofthe following claims.

In conjunction with the above disclosure, we claim:
 1. An implantableanti-arrhythmia device comprising:means for defining a first set ofarrhythmia detection criteria:means for monitoring a patients heartrhythm; means for determining whether the patient's heart rhythm meetsthe first set of criteria; means in responsive to a determination thatthe patient's heart rhythm meets the first set of criteria fordelivering a first anti-arrhythmia therapy; means for defining a secondset of arrhythmia detection criteria less stringent than the first setof criteria; means for receiving a patient activation signal; meansresponsive to receipt of a patient activation signal for determiningwhether the patient's heart rhythm meets the second set of criteria; andmeans responsive to a determination that the patient's heart rhythmmeets the second set of criteria, for delivering a secondanti-arrhythmia therapy.
 2. A device according to claim 1 wherein themeans for defining a second set of criteria comprises means for defininga set of criteria which are a subset of the first set of criteria.
 3. Adevice according to claim 1 wherein the first and second therapiesdiffer from one another.
 4. An implantable anti-arrhythmia devicecomprising:means for defining a first set of arrhythmia detectioncriteria:means for monitoring a patients heart rhythm; means fordetermining whether the patient's heart rhythm meets the first set ofcriteria; means in responsive to a determination that the patient'sheart rhythm meets the first set of criteria for delivering a firstanti-arrhythmia therapy; means for defining a second set of arrhythmiadetection criteria; means for receiving a patient activation signal;means responsive to receipt of a patient activation signal fordetermining whether the patient's heart rhythm meets the second set ofcriteria; and means responsive to a determination that the patient'sheart rhythm meets the second set of criteria for delivering a secondanti-arrhythmia therapy; and wherein the means for determining whetherthe patient's heart rhythm meets the second set of criteria comprisesmeans for determining whether the patient's heart rhythm meets thesecond set of criteria within a predetermined time interval followingreceipt of a patient activation signal.
 5. A device according to claim 4further comprising means for defining a third set of arrhythmiadetection criteria and means responsive to the patient's heart rhythmmeeting the third set of criteria at time of receipt of the patientactivation signal for enabling the means for determining whether thepatient's heart rhythm meets the second set of criteria.
 6. A deviceaccording to claim 5 wherein the means for defining a third set ofarrhythmia detection criteria comprises means for defining a third setof criteria less stringent than the second set of criteria.
 7. Animplantable anti-arrhythmia device comprising:means for defining a firstset of arrhythmia detection criteria:means for monitoring a patientsheart rhythm; means for determining whether the patient's heart rhythmmeets the first set of criteria; means in responsive to a determinationthat the patient's heart rhythm meets the first set of criteria fordelivering a first anti-arrhythmia therapy; means for defining a secondset of arrhythmia detection criteria; means for receiving a patientactivation signal; means responsive to receipt of a patient activationsignal for determining whether the patient's heart rhythm meets thesecond set of criteria; and means responsive to a determination that thepatient's heart rhythm meets the second set of criteria, for deliveringa second anti-arrhythmia therapy; and further comprising means fortransmitting information to an external receiver indicative of whetherthe second therapy is to be delivered in response to the patientactivation signal.
 8. An implantable anti-arrhythmia devicecomprising:means for defining a first set of arrhythmia detectioncriteria:means for monitoring a patients heart rhythm; means fordetermining whether the patient's heart rhythm meets the first set ofcriteria; means in responsive to a determination that the patient'sheart rhythm meets the first set of criteria for delivering a firstanti-arrhythmia therapy; means for defining a second set of arrhythmiadetection criteria; means for receiving a patient activation signal;means responsive to receipt of a patient activation signal fordetermining whether the patient's heart rhythm meets the second set ofcriteria; and means responsive to a determination that the patient'sheart rhythm meets the second set of criteria, for delivering a secondanti-arrhythmia therapy; and further comprising means for transmittinginformation to an external receiver indicative that the second therapywill not be delivered in response to the patient activation signal.